fixes, safeguards and docs for "compile-time arithmetics"

crsn_arith/src/exec.rs

@@ -757,6 +757,15 @@ impl OpTrait for ArithOp {
             ArithOp::FloatHypAcot { dst, a } => to_sexp_1_or_2("facoth", dst, a),
         }
     }
+
+    fn is_compile_time_evaluable(&self) -> bool {
+        match self {
+            ArithOp::Test { .. } | ArithOp::Compare { .. } | ArithOp::RangeTest { .. }
+            | ArithOp::FloatTest { .. } | ArithOp::FloatCompare { .. } | ArithOp::FloatRangeTest { .. } => false,
+            // rng is allowed... makes little sense, but it also has no side effects
+            _ => true, // a bold claim...
+        }
+    }
 }
 
 fn to_sexp_2_or_3(name: &str, dst: &Wr, a: &Rd, b: &Rd) -> Sexp {