dereference first argument of (is)/2
@@ -597,7 +597,7 @@ fn call_builtin(
return_from_clause!(machine_st.last_call, machine_st)
}
&BuiltInClauseType::Is(r, ref at) => {
- let n1 = machine_st[r];
+ let n1 = machine_st.store(machine_st.deref(machine_st[r]));
let n2 = machine_st.get_number(at)?;
match n2 {
