Parsing of '(\)' set difference operator fails

[ggeier]

guarantee {
    ||[i IN ({a(), b(), c()} (\) {b(), d()})] [out <- i];
}

results in the following error (tslcheck, tslresolve, etc):

"Syntax Error" (line 2, column 31):
unexpected "\\"
expecting "(", "{", "|", "otherwise", "false", "true", "_", digit, "[" or identifier