Function eval_expression_function 

pub fn eval_expression_function<'a, 'arena>(
    ast: &'arena AstExpr<'arena>,
    param_names: &[Cow<'a, str>],
    arg_values: &[Real],
    parent_ctx: Option<Rc<EvalContext>>,
    arena: &'arena Bump,
) -> Result<Real>

Evaluates an expression function with the given arguments.

This is a helper function used internally by the evaluation logic.