public CFGBuilder assign(DfaValue target, DfType source) {
    if (target instanceof DfaVariableValue) {
      if (source == DfType.TOP) {
        flush(target).push(target);
      } else {
        push(source).assignTo((DfaVariableValue)target);
      }
    } else {
      push(source);
    }
    return this;
  }
public CFGBuilder assign(DfaValue target, DfType source)