HashMap<String, SemanticType> environment,
HashSet<String> generics, WyalFile.Context context) {
environment = new HashMap<String,SemanticType>(environment);
propagate(e.pattern,environment,generics,context);
SemanticType r = propagate(e.operand,environment,generics,context);
checkIsSubtype(SemanticType.Bool,r,e.operand);
return SemanticType.Bool;
}