// e.g. hasWord($x) => bla
// to:  for $x where hasWord($x): bla

sO ai_lhsToForWhere(BasicLogicRule rule, DeepZipTools tools) {
  ret ai_lhsToForWhere(rule, null, tools);
}

sO ai_lhsToForWhere(BasicLogicRule rule, Set knownVars, DeepZipTools tools) {
  rule = (BasicLogicRule) ai_fragmentExpressionsToFunctionCalls(rule, tools);
  if (!(rule.lhs instanceof FunctionCall)) ret rule;
  FunctionCall fc = (FunctionCall) rule.lhs;
  if (l(fc.args) != 1) ret rule;
  O var = first(fc.args);
  if (!tools.isVar(var) || contains(knownVars, var)) ret rule;
  ret ForWhere(var, rule.lhs, rule.rhs);
}