Function integrate_definite 

pub fn integrate_definite(
    expr: &Expr,
    x: &str,
    a: f64,
    b: f64,
    steps: usize,
) -> Option<f64>

Definite integral ∫_a^b expr dx via the Fundamental Theorem when an antiderivative exists, else a numerical Simpson fallback over steps (even) panels.