/* Shared lesson stylesheet — first component of the course.
   Every lesson links this so lessons look like one course. */
:root {
  --paper: #fdfcf8;
  --ink: #1a1a1a;
  --muted: #6b675f;
  --accent: #1a5fb4;
  --rule: #e5e0d5;
  --box: #f6f3ec;
  --green: #1e7e34;
  --red: #b42318;
  --serif: "Iowan Old Style", "Palatino Linotype", Palatino, Georgia, serif;
  --sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Inter, sans-serif;
  --mono: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
}
* { box-sizing: border-box; }
body {
  background: var(--paper);
  color: var(--ink);
  font-family: var(--serif);
  line-height: 1.6;
  margin: 0;
  padding: 2rem 1rem 4rem;
}
article {
  max-width: 720px;
  margin: 0 auto;
}
nav.top {
  font-family: var(--sans);
  font-size: 0.85rem;
  color: var(--muted);
  border-bottom: 1px solid var(--rule);
  padding-bottom: 0.75rem;
  margin-bottom: 2rem;
  display: flex;
  gap: 1rem;
  flex-wrap: wrap;
}
nav.top a { color: var(--accent); text-decoration: none; }
h1 { font-size: 2rem; line-height: 1.2; margin: 0 0 0.25rem; letter-spacing: -0.01em; }
.subtitle { color: var(--muted); font-family: var(--sans); font-size: 0.95rem; margin-bottom: 2rem; }
h2 { font-size: 1.25rem; margin: 2.2rem 0 0.6rem; border-top: 1px solid var(--rule); padding-top: 1.2rem; }
p, li { font-size: 1.05rem; }
.muted { color: var(--muted); }
.defbox, .example, .takeaway {
  background: var(--box);
  border: 1px solid var(--rule);
  border-left: 4px solid var(--accent);
  border-radius: 6px;
  padding: 1rem 1.25rem;
  margin: 1.25rem 0;
}
.example { border-left-color: #8a6d1b; }
.takeaway { border-left-color: var(--green); }
.eq { overflow-x: auto; text-align: center; padding: 0.5rem 0; }
.eqnum { color: var(--muted); font-family: var(--sans); font-size: 0.8rem; }
table.demo { border-collapse: collapse; margin: 1rem auto; font-family: var(--sans); font-size: 0.95rem; }
table.demo th, table.demo td { border: 1px solid var(--rule); padding: 0.45rem 0.8rem; text-align: center; }
table.demo th { background: var(--box); }
code, .mono { font-family: var(--mono); font-size: 0.92em; }
.quiz { border: 1px solid var(--rule); border-radius: 8px; padding: 1rem 1.25rem; margin: 1rem 0; background: #fff; }
.quiz fieldset { border: none; padding: 0; margin: 0; }
.quiz legend { font-family: var(--sans); font-weight: 600; padding: 0; margin-bottom: 0.5rem; }
.quiz label {
  display: block;
  font-family: var(--mono);
  background: var(--paper);
  border: 1px solid var(--rule);
  border-radius: 6px;
  padding: 0.45rem 0.7rem;
  margin: 0.4rem 0;
  cursor: pointer;
}
.quiz label:hover { border-color: var(--accent); }
.quiz .feedback { font-family: var(--sans); font-size: 0.9rem; margin-top: 0.5rem; min-height: 1.4em; }
.quiz .feedback.ok { color: var(--green); }
.quiz .feedback.no { color: var(--red); }
.cols { display: grid; grid-template-columns: 1fr 1fr; gap: 1rem; }
@media (max-width: 600px) { .cols { grid-template-columns: 1fr; } }
.card { background: #fff; border: 1px solid var(--rule); border-radius: 8px; padding: 0.9rem 1rem; }
.card h3 { margin: 0 0 0.4rem; font-family: var(--sans); font-size: 0.95rem; }
/* Proof-flow: vertical memorize-steps with write/say split */
.flow { display: flex; flex-direction: column; gap: 0; margin: 1.25rem 0; }
.fstep { background: #fff; border: 1px solid var(--rule); border-radius: 8px; padding: 0.8rem 1rem; }
.fstep h3 { margin: 0 0 0.35rem; font-family: var(--sans); font-size: 0.95rem; }
.fstep h3 .n { display: inline-block; background: var(--ink); color: #fff; font-size: 0.75rem; width: 1.4em; height: 1.4em; line-height: 1.4em; text-align: center; border-radius: 50%; margin-right: 0.45rem; vertical-align: 1px; }
.fstep.branch-a { border-left: 4px solid var(--accent); }
.fstep.branch-b { border-left: 4px solid #8a6d1b; }
.fwrite { font-family: var(--mono); font-size: 0.92rem; background: var(--box); border-radius: 6px; padding: 0.4rem 0.6rem; margin: 0.4rem 0; overflow-x: auto; }
.fsay { font-size: 0.98rem; margin: 0.3rem 0 0; }
.fsay strong { font-family: var(--sans); font-size: 0.8rem; text-transform: uppercase; letter-spacing: 0.04em; color: var(--muted); }
.farrow { text-align: center; color: var(--muted); font-size: 1.2rem; line-height: 1; padding: 0.15rem 0; }
.memo { font-family: var(--sans); font-size: 0.9rem; background: #eef4ff; border: 1px dashed var(--accent); border-radius: 8px; padding: 0.7rem 1rem; margin: 1rem 0; }
footer { margin-top: 3rem; border-top: 1px solid var(--rule); padding-top: 1rem; font-family: var(--sans); font-size: 0.85rem; color: var(--muted); }
a { color: var(--accent); }
/* Sidebar layout — shared course navigation */
.layout { display: flex; gap: 2rem; max-width: 1020px; margin: 0 auto; align-items: flex-start; }
.layout > article { flex: 1; min-width: 0; margin: 0; max-width: 720px; }
.sidebar {
  width: 230px; flex-shrink: 0;
  position: sticky; top: 1rem;
  font-family: var(--sans); font-size: 0.9rem;
  background: var(--box); border: 1px solid var(--rule); border-radius: 8px;
  padding: 1rem;
  max-height: calc(100vh - 2rem); overflow-y: auto;
}
.sidebar .brand { display: block; font-weight: 700; color: var(--ink); text-decoration: none; margin-bottom: 0.5rem; }
.sidebar .side-label { display: block; font-size: 0.75rem; text-transform: uppercase; letter-spacing: 0.05em; color: var(--muted); margin: 0.5rem 0 0.4rem; }
.sidebar a.lesson { display: block; color: var(--accent); text-decoration: none; padding: 0.35rem 0.5rem; border-radius: 6px; margin: 0.15rem 0; }
.sidebar a.lesson:hover { background: #fff; }
.sidebar a.lesson[aria-current="page"] { background: #fff; color: var(--ink); font-weight: 600; border: 1px solid var(--rule); }
@media (max-width: 820px) {
  body { padding: 1rem 0.75rem 3rem; }
  .layout { flex-direction: column; }
  .sidebar { width: 100%; position: static; max-height: none; }
}
@media print { body { background: #fff; } .quiz label { background: #fff; } }