Function missingIfEmpty

src/math-json/utils.ts:459–463

(
  expr: MathJsonExpression | null | undefined
)

}

export function missingIfEmpty(
  expr: MathJsonExpression | null | undefined
): MathJsonExpression {
  return isEmptySequence(expr) ? MISSING : expr;
}

function countFunctionLeaves(xs: MathJsonExpression[]): number {
  if (xs[0] === 'Square') {