Function builtin_factorial 

fn builtin_factorial(
    args: &[RValue],
    _: &[(String, RValue)],
) -> Result<RValue, RError>

Factorial: n! = 1 * 2 * … * n.

Uses gamma(n + 1) for non-integer and large values.

@param x numeric vector (non-negative) @return numeric vector of factorials