public CFGBuilder assignAndPop(DfaValue target, DfType source) {
    if (target instanceof DfaVariableValue) {
      if (source == DfType.TOP) {
        add(new FlushVariableInstruction((DfaVariableValue)target));
      } else {
        push(source).assignTo((DfaVariableValue)target).pop();
      }
    }
    return this;
  }
public CFGBuilder assignAndPop(DfaValue target, DfType source)