Function simplify 

pub fn simplify(
    pool: &ExprPool,
    builtins: &BuiltinSymbols,
    id: ExprId,
) -> ExprId

Full simplification (spec §8.3 levels 2/3): fold recursively, then apply rules on demand — Pow(sqrt(x), 2) → x, Euler’s e^{iθ} → cos + i·sin, constant folding for sin/cos/exp/log/ln/abs/sqrt, Pow(x, 1/2) → \sqrt{x}. Simplification never changes the mathematical value.