Function builtin_as_call 

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

Convert a list to a call object (Language).

as.call(list(f, a, b)) creates the call f(a, b). The first element is the function, remaining are arguments.

@param x a list @return a call (Language) object @namespace base