Function integrate_cas 

pub fn integrate_cas(
    cx: &mut Cx,
    expr: &CasExpr,
    var: &Symbol,
) -> Result<CasExpr>

Integrate a CasExpr with respect to var, returning a simplified antiderivative tree (without constant of integration).

Covers the builtin arithmetic operators and power-of-variable cases; inputs outside the supported set produce an error.