condition function
condition — empty Str → false; returns bool on success, false on failure.
Implementation
bool condition(String str) {
if (str.isEmpty) return false;
final ev = _currentEvaluator;
if (ev == null) return false;
final r = ev.cond(str);
if (ev.hasError) {
// ignore: avoid_print
print('<font color="#ff0000">[ERROR] $str # ${ev.lastError}</font>');
return false;
}
return r;
}