Function holdMap

src/compute-engine/boxed-expression/hold.ts:13–46

(
  expr: Expression,
  f: (x: Expression) => Expression | null
)

 * If `f` returns `null`, the element is not added to the result
 */
export function holdMap(
  expr: Expression,
  f: (x: Expression) => Expression | null
): ReadonlyArray<Expression> {
  if (!isFunction(expr)) return [];

  let xs = expr.ops;

  const def = expr.operatorDefinition;

  if (!def || xs.length === 0) return xs;

  // f(a, f(b, c), d) -> f(a, b, c, d)
  // Ellipsis fold barrier: a `ContinuationPlaceholder` operand marks a
  // notational sum/product; do not lift nested associative operands (it would
  // tear a coefficient out of an anchor like the `2n` in `Multiply(2, n)`).
  const hasContinuation = xs.some((x) => isContinuationOperand(x));
  if (def?.associative && !hasContinuation)
    xs = flatten(xs, expr.operator, false);

  //
  // Apply the hold as necessary
  //
  if (def.lazy) return xs;

  const result: Expression[] = [];
  for (const x of xs) {
    const h = x.operator;
    if (h === 'Hold') result.push(x);
    else {
      const op = h === 'ReleaseHold' && isFunction(x) ? x.op1 : x;
      if (op) {
        const y = f(op);
        if (y !== null) result.push(y);
      }
    }
  }