Function angleToRadians

src/compute-engine/boxed-expression/utils.ts:167–177

(x: Expression | undefined)

  for (const e of exprs)
    for (const n of e.unknowns) if (n !== '_') names.add(n);
  if (names.size === 1) return names.values().next().value;
  if (names.size > 1 && names.has('x')) return 'x';
  return undefined;
}

/**
 * Operator heads whose evaluation is a pure expression-transformation step:
 * the result is an expression in the same free variables — no symbol-value
 * substitution, no relational collapse.
 *
 * A structural algorithm that *holds* its expression operand (`Solve`,
 * `Integrate`, `Limit`, …) should reduce such a head before running:
 * `Solve(Simplify(eq), x)` means "simplify, then solve", not "solve an
 * expression whose operator is `Simplify`" (which finds no roots). This is