Function interp_set_validity 

fn interp_set_validity(
    args: &[RValue],
    named: &[(String, RValue)],
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Set a validity method for an S4 class.

Stores the validity function in the class registry entry.

@param Class character string naming the class @param method validity-checking function @return the class name