/* PE3101P week 2 -- web edition.
   Follows tomifrancis.com: Lato, teal #006580, white.
   Generated pages read this; nothing here is page-specific. */

:root {
  --ink: #12222b;
  --ink-soft: #46606c;
  --teal: #006580;
  --teal-soft: #e6f2f5;
  --rule: #dde6ea;
  --bg: #ffffff;
  --paper: #fafcfd;
  --accent: #b8860b;
  --fig-bg: #ffffff;
  --fig-faint: #9db4bd;
  --maxw: 43rem;
  --peek-w: 20.25rem;
  --paper-peek: #f7fafb;
}

* { box-sizing: border-box; }

html { -webkit-text-size-adjust: 100%; }

body {
  margin: 0;
  /* ⚠️ THE DEFINITION PANEL'S COLUMN IS RESERVED PERMANENTLY, open or not.
     Tomi's rule from the primer, 17 August 2026: "if you click on a definition,
     the main text doesn't move at all. (Currently it resizes, which is not
     great GUI)." When nothing is open the gutter is simply blank.
     ⛔ .peekopen must therefore have NO layout effect whatsoever. */
  padding-right: var(--peek-w);
  background: var(--bg);
  color: var(--ink);
  font-family: Lato, "Segoe UI", system-ui, sans-serif;
  font-size: 17px;
  line-height: 1.62;
}

.skip {
  position: absolute; left: -9999px;
}
.skip:focus {
  left: 1rem; top: 1rem; z-index: 100; background: var(--teal);
  color: #fff; padding: .5rem .9rem; border-radius: 4px;
}

/* ---------------------------------------------------------------- layout */
.layout {
  display: grid;
  grid-template-columns: 17rem minmax(0, 1fr);
  gap: 2.5rem;
  max-width: 76rem;
  margin: 0 auto;
  padding: 0 1.5rem;
}

main {
  min-width: 0;
  padding: 2.6rem 0 6rem;
  max-width: var(--maxw);
}

/* ---------------------------------------------------------------- toc */
.toc {
  position: sticky;
  top: 0;
  align-self: start;
  max-height: 100vh;
  overflow-y: auto;
  padding: 2.6rem .5rem 3rem 0;
  font-size: 14.5px;
  border-right: 1px solid var(--rule);
}
.toctitle {
  font-weight: 700; letter-spacing: .01em;
  margin-bottom: .9rem; font-size: 15px;
}
/* The title is the way back to the contents page. */
.toctitle a {
  color: var(--teal); text-decoration: none; display: block;
  padding: .15rem .4rem; margin-left: -.4rem; border-radius: 4px;
}
.toctitle a:hover { background: var(--teal-soft); }
.toc ol { list-style: none; margin: 0; padding: 0; }
.toc > ol > li { margin: .2rem 0; }
.tocnum {
  color: var(--ink-soft); margin-right: .45rem; font-variant-numeric: tabular-nums;
}
.tocnum::after { content: "."; }
.toc li.here > a .tocnum { color: var(--teal); }
.toc a {
  color: var(--ink); text-decoration: none; display: block;
  padding: .22rem .5rem .22rem .4rem; border-radius: 4px; line-height: 1.35;
}
.toc a:hover { background: var(--teal-soft); color: var(--teal); }
.toc li.here > a { color: var(--teal); font-weight: 700; }
.toc ul {
  list-style: none; margin: .25rem 0 .6rem; padding-left: 1.1rem;
  border-left: 2px solid var(--teal-soft);
}
.toc ul a { font-size: 13.6px; color: var(--ink-soft); padding: .15rem .4rem; }
/* The subsection you are currently on. Set by spy() in notes.js; see the note
   there for Tomi's rule about which subsection counts as current. The marker
   sits in the padding so the text does not shift as the highlight moves. */
.toc ul a.active {
  color: var(--teal); font-weight: 700; background: var(--teal-soft);
  border-radius: 3px; position: relative;
}
.toc ul a.active::before {
  content: ""; position: absolute; left: -1.1rem; top: 0; bottom: 0;
  width: 2px; background: var(--teal);
}

/* ---------------------------------------------------------------- headings */
h1 {
  font-size: 1.85rem; line-height: 1.2; margin: 0 0 1.6rem;
  font-weight: 700; letter-spacing: -.01em;
}
h1 .secnum, .contents .secnum {
  color: var(--teal); margin-right: .55rem; font-variant-numeric: tabular-nums;
}
.subhead {
  font-size: 1.16rem; margin: 2.4rem 0 .7rem; font-weight: 700;
  line-height: 1.3; scroll-margin-top: 1.5rem;
}
.subhead .secnum {
  color: var(--teal); font-size: .82em; margin-right: .5rem;
  font-variant-numeric: tabular-nums; font-weight: 700;
}
.anchor {
  opacity: 0; margin-left: .45rem; color: var(--teal); text-decoration: none;
  font-weight: 400;
}
.subhead:hover .anchor { opacity: .55; }
.anchor:hover { opacity: 1 !important; }

p { margin: 0 0 .95rem; }

/* ---------------------------------------------------------------- lists */
ul, ol { margin: 0 0 1rem; padding-left: 1.4rem; }
li { margin: .3rem 0; }
ul.customlist, ol.customlist { list-style: none; padding-left: .5rem; }
.customlist .itemlabel {
  color: var(--teal); font-weight: 700; margin-right: .35rem;
  font-variant-numeric: tabular-nums;
}

/* ---------------------------------------------------------------- boxes */
.box {
  border: 1px solid var(--rule);
  border-left: 3px solid var(--teal);
  border-radius: 5px;
  background: var(--paper);
  padding: .85rem 1.1rem .5rem;
  margin: 1.3rem 0;
}
.boxhead {
  font-weight: 700; color: var(--teal); font-size: .93rem;
  letter-spacing: .02em; margin-bottom: .35rem;
}
.boxname { font-weight: 400; color: var(--ink-soft); }
.box-exercise { border-left-color: var(--accent); background: #fdfaf3; }
.box-exercise .boxhead { color: #8a6508; }
.box-example { border-left-color: #5a7d8c; }
.box-example .boxhead { color: #4a6b78; }
.box p:last-child { margin-bottom: .5rem; }

/* ---------------------------------------------------------------- figures */
figure.figure {
  margin: 1.6rem 0; text-align: center;
  border: 1px solid var(--rule); border-radius: 6px;
  padding: 1.1rem .8rem .8rem; background: var(--paper);
}
svg.fig { max-width: 100%; height: auto; color: var(--ink); }
figcaption {
  font-size: .88rem; color: var(--ink-soft); margin-top: .7rem;
  text-align: left; line-height: 1.5;
}
.fignum { font-weight: 700; color: var(--teal); }
text.figlbl { font-family: Georgia, "Times New Roman", serif; fill: currentColor; }
text.figkey { font-family: Georgia, "Times New Roman", serif; fill: currentColor; }
text.faint { fill: var(--fig-faint); }

/* ---------------------------------------------------------------- tables */
.tablewrap { overflow-x: auto; margin: 1.3rem 0; }
table.tbl {
  border-collapse: collapse; margin: 0 auto; font-size: .95rem;
}
table.tbl th, table.tbl td { padding: .4rem .85rem; }
table.tbl th {
  border-top: 2px solid var(--ink); border-bottom: 1px solid var(--ink);
  font-weight: 700;
}
table.tbl tr:last-child td { border-bottom: 2px solid var(--ink); }
table.tbl .align-left { text-align: left; }
table.tbl .align-center { text-align: center; }
table.tbl .align-right { text-align: right; }
table.tbl .rule { border-left: 1px solid var(--rule); }

/* ---------------------------------------------------------------- maths */
.mathblock { margin: 1.1rem 0; overflow-x: auto; overflow-y: hidden; padding: .2rem 0; }
.katex { font-size: 1.04em; }
.katex-display { margin: 0; }
.math-fallback {
  font-family: ui-monospace, Menlo, Consolas, monospace;
  font-size: .9em; color: var(--ink-soft);
}
.math-error { color: #a33; border-bottom: 1px dotted #a33; }

/* ---------------------------------------------------------------- misc */
a { color: var(--teal); }
a.url { word-break: break-word; }
.xref { text-decoration: none; border-bottom: 1px solid var(--teal-soft); }
.xref:hover { border-bottom-color: var(--teal); }
.xref.broken { color: #a33; border-bottom: 1px dotted #a33; }
code { font-family: ui-monospace, Menlo, Consolas, monospace; font-size: .92em; }
.sc { font-variant: small-caps; }

.footnotes {
  margin-top: 3rem; padding-top: .8rem; border-top: 1px solid var(--rule);
  font-size: .88rem; color: var(--ink-soft);
}
.fnref a { text-decoration: none; }
.fnback { text-decoration: none; margin-left: .3rem; }

.pagenav {
  display: flex; justify-content: space-between; gap: 1rem;
  margin-top: 3.5rem; padding-top: 1.1rem; border-top: 1px solid var(--rule);
  font-size: .93rem;
}
.pagenav a { text-decoration: none; max-width: 48%; }
.pagenav a:hover { text-decoration: underline; }
.pagenav .next { margin-left: auto; text-align: right; }

/* ---------------------------------------------------------------- landing */
.landing .lede { color: var(--ink-soft); font-size: 1.05rem; margin-bottom: 1.6rem; }
.usenote {
  background: var(--teal-soft); border-radius: 6px; padding: .9rem 1.2rem;
  font-size: .93rem; margin-bottom: 2rem;
}
.usenote p { margin: .35rem 0; }
ol.contents { list-style: none; padding: 0; counter-reset: c; }
ol.contents li {
  counter-increment: c; padding: .7rem 0; border-bottom: 1px solid var(--rule);
}
ol.contents a { text-decoration: none; font-weight: 700; font-size: 1.06rem; }
ol.contents a:hover { text-decoration: underline; }
.subsList { color: var(--ink-soft); font-size: .87rem; margin-top: .2rem; }

/* ---------------------------------------------------------------- responsive */
@media (max-width: 940px) {
  .layout { grid-template-columns: 1fr; gap: 0; padding: 0 1.1rem; }
  .toc {
    position: static; max-height: none; border-right: 0;
    border-bottom: 1px solid var(--rule); padding: 1.2rem 0;
  }
  main { padding-top: 1.6rem; }
  body { font-size: 16.5px; }
}

/* --------------------------------------------------- collapsible structure */
/* Tomi, 21 Aug 2026: "Expandability/collapsibility should be nested. It should
   be possible to expand or collapse a subsection (in general). Subsections
   should normally be expanded by default, but it should be possible to flag any
   collapsible object as collapsed by default."  The flag is COLLAPSED_BY_DEFAULT
   in build/build_site.py; it takes subsection ids ("3.2") and figure names. */

details.subdet, details.figdet { display: block; }

summary.subhead {
  cursor: pointer;
  list-style: none;
  display: flex;
  align-items: baseline;
  gap: .1rem;
}
summary.subhead::-webkit-details-marker { display: none; }
summary.subhead::before {
  content: "\25be";                     /* filled down-triangle */
  color: var(--teal);
  font-size: .8em;
  width: .9em;
  flex: 0 0 auto;
  transition: transform .12s;
}
details.subdet:not([open]) > summary.subhead::before { transform: rotate(-90deg); }
details.subdet:not([open]) > summary.subhead { color: var(--ink-soft); }
details.subdet:not([open]) > summary.subhead::after {
  content: "collapsed";
  font-size: .68rem; font-weight: 400; letter-spacing: .05em;
  text-transform: uppercase; color: var(--ink-soft);
  border: 1px solid var(--rule); border-radius: 3px;
  padding: 0 .3rem; margin-left: .5rem; align-self: center;
}
.subbody { padding-top: .1rem; }

/* figures: the caption is the summary, so a collapsed figure still says what it
   is rather than leaving an unexplained gap */
details.figdet > summary {
  cursor: pointer;
  list-style: none;
  font-size: .88rem; color: var(--ink-soft); line-height: 1.5;
  text-align: left;
}
details.figdet > summary::-webkit-details-marker { display: none; }
details.figdet > summary::before {
  content: "\25be";
  color: var(--teal); font-size: .85em; margin-right: .35rem;
  display: inline-block; transition: transform .12s;
}
details.figdet:not([open]) > summary::before { transform: rotate(-90deg); }
details.figdet[open] > summary { order: 2; margin-top: .7rem; }
details.figdet[open] { display: flex; flex-direction: column; }
.figbody { order: 1; }

.usenote.archived { background: #fdf3e6; }

/* ------------------------------------------- check-your-understanding boxes */
/* Tomi, 21 Aug 2026: questions to the student, with the answer hidden behind a
   coloured toggle. The whole box is collapsed by default so it never gets in
   the way of a first read. */
.qbox { margin: 1.4rem 0; }
details.qdet {
  border: 1px solid var(--rule); border-left: 3px solid var(--teal);
  border-radius: 5px; background: var(--paper); padding: .1rem 0;
}
details.qdet > summary {
  cursor: pointer; list-style: none;
  padding: .6rem .9rem; font-weight: 700; color: var(--teal);
  display: flex; align-items: center; gap: .55rem;
}
details.qdet > summary::-webkit-details-marker { display: none; }
.qmark {
  flex: 0 0 auto; width: 1.35rem; height: 1.35rem; border-radius: 50%;
  background: var(--teal); color: #fff; font-size: .82rem;
  display: flex; align-items: center; justify-content: center;
}
details.qdet[open] > summary { border-bottom: 1px solid var(--rule); }
ol.qlist { margin: .7rem 0 .9rem; padding-left: 2.4rem; }
li.qitem { margin: .55rem 0; }
.qtext { margin-bottom: .25rem; }

details.qans { display: inline-block; }
details.qans > summary {
  cursor: pointer; list-style: none;
  color: var(--accent); font-weight: 700; font-size: .88rem;
  border-bottom: 1px dotted var(--accent); display: inline-block;
}
details.qans > summary::-webkit-details-marker { display: none; }
details.qans[open] > summary { color: var(--ink-soft); border-bottom-color: transparent; }
details.qans[open] > summary::after { content: "\00a0\2014\00a0hide"; font-weight: 400; }
.qbody {
  margin-top: .3rem; padding: .45rem .7rem;
  background: #fff; border-left: 2px solid var(--accent); border-radius: 0 4px 4px 0;
}
.qbody p:last-child { margin-bottom: 0; }

/* ---------------------------------------------- optional / non-examinable */
.aside { margin: 1.6rem 0; }
details.asidedet {
  border: 1px dashed var(--rule); border-radius: 6px; background: #fcfcfd;
}
details.asidedet > summary {
  cursor: pointer; list-style: none;
  padding: .6rem .9rem; font-weight: 700; color: var(--ink-soft);
  display: flex; align-items: center; gap: .6rem;
}
details.asidedet > summary::-webkit-details-marker { display: none; }
.optmark {
  flex: 0 0 auto; font-size: .68rem; letter-spacing: .06em; text-transform: uppercase;
  background: var(--ink-soft); color: #fff; border-radius: 3px; padding: .1rem .4rem;
}
details.asidedet[open] > summary { border-bottom: 1px solid var(--rule); }
.asidebody { padding: .8rem 1.1rem .2rem; font-size: .95rem; }
.asidebody .mathblock { font-size: .95em; }

/* --------------------------------------------- stepped figure sequences */
.figseq { margin: 1.6rem 0; }
.figseq .seqframes { position: relative; }
.figseq figure.figure { margin: 0; }
.seqbar {
  display: flex; align-items: center; justify-content: center; gap: .9rem;
  margin-top: .5rem;
}
.seqbtn {
  font: inherit; font-size: 1rem; line-height: 1; cursor: pointer;
  background: var(--paper); color: var(--teal);
  border: 1px solid var(--rule); border-radius: 5px;
  padding: .3rem .75rem;
}
.seqbtn:hover:not(:disabled) { background: var(--teal-soft); border-color: var(--teal); }
.seqbtn:disabled { color: var(--rule); cursor: default; }
.seqcount {
  font-size: .85rem; color: var(--ink-soft); font-variant-numeric: tabular-nums;
  min-width: 3.2rem; text-align: center;
}
.seqcap {
  font-size: .88rem; color: var(--ink-soft); text-align: center;
  margin: .4rem auto 0; max-width: 34rem;
}
.figseq:focus-visible { outline: 2px solid var(--teal); outline-offset: 4px; }

/* -------------------------------------------------- interactive figures */
.widget-rot svg { display: block; max-width: 340px; margin: 0 auto; }
.widget-ctl {
  display: flex; align-items: center; justify-content: center;
  gap: .8rem; flex-wrap: wrap; margin-top: .5rem; font-size: .88rem;
}
.widget-ctl label { display: flex; align-items: center; gap: .45rem; color: var(--ink-soft); }
.widget-ctl input[type="range"] { width: 12rem; accent-color: var(--teal); }
.widget-ctl output {
  font-variant-numeric: tabular-nums; color: var(--teal); font-weight: 700;
  min-width: 6.2rem;
}

/* ------------------------------------------- defined terms + the panel */
/* Restrained on purpose: with "event", "union" and "intersection" all live,
   colouring every one would turn the page blue. A dotted rule that only picks
   up colour on hover, as in the primer. */
a.xref.term {
  color: inherit; text-decoration: none; cursor: pointer;
  border-bottom: 1px dotted rgba(0, 101, 128, .55);
}
a.xref.term:hover { color: var(--teal); background: var(--teal-soft); }

.peek {
  position: fixed; top: 0; right: 0; z-index: 40;
  width: var(--peek-w); height: 100vh;
  background: var(--paper-peek);
  border-left: 1px solid var(--rule);
  box-shadow: -8px 0 28px rgba(0, 0, 0, .07);
  display: flex; flex-direction: column;
  animation: peekin .16s ease;
}
@keyframes peekin { from { transform: translateX(12px); opacity: 0; } }
.peek[hidden] { display: none; }
/* Grid, not a flex row: "mutually exclusive / disjoint" wraps, and the close
   button must stay pinned top-right while the jump link sits underneath. */
.peekbar {
  display: grid; grid-template-columns: 1fr auto;
  gap: .2rem .6rem; align-items: start;
  padding: .7rem .9rem; border-bottom: 1px solid var(--rule); background: #fff;
}
.peektitle {
  grid-column: 1; grid-row: 1;
  font-weight: 700; font-size: .78rem; letter-spacing: .06em;
  text-transform: uppercase; color: var(--teal); line-height: 1.35;
}
.peekbar .peekclose { grid-column: 2; grid-row: 1; }
.peekbar .peekgo { grid-column: 1 / -1; grid-row: 2; justify-self: start; }
.peekgo {
  font-size: .8rem; color: var(--teal); text-decoration: none; white-space: nowrap;
}
.peekgo:hover { text-decoration: underline; }
.peekclose {
  font: inherit; font-size: 1.4rem; line-height: 1; cursor: pointer;
  background: none; border: 0; color: var(--ink-soft); padding: 0 .2rem;
}
.peekclose:hover { color: var(--ink); }
.peekbody {
  flex: 1 1 auto; overflow-y: auto; padding: .9rem;
  font-size: .95rem;
  /* ⛔ Never a sideways scrollbar here (Tomi, 22 Aug 2026). notes.js scales the
     inner block down if it would overrun; this is the belt to that's braces. */
  overflow-x: hidden;
}
/* notes.js shrinks this until nothing inside it overflows. ⛔ Do not give it a
   transform: a visual scale leaves the .mathblock overflowing and scrolling. */
.peekfit { font-size: 100%; }
.peekbody .blk { margin-top: 0; }
.peekbody .box { margin: 0; }
/* The panel is a quotation of the page, not a second interactive copy. */
.peekbody .anchor, .peekbody .cmt-tools, .peekbody .cmt-badge { display: none; }

/* Where "Go to" lands. */
.flash { animation: flash 1.2s ease; }
@keyframes flash {
  from { background: #fdf3d3; } to { background: transparent; }
}

/* the heading is a commentable block; keep the toolbar off the text */
summary.subhead.blk { padding-right: 7rem; }
.headtarget { display: block; height: 0; scroll-margin-top: 1.5rem; }
h1.blk { padding-right: 7rem; }