code: protected boolean isWellDefined() {
      int size = references.size();
      if (size == 0) {
        return false;
      }
  
      // If this is a declaration that does not instantiate the variable,
      // it's not well-defined.
      Reference init = getInitializingReference();
      if (init == null) {
        return false;
      }
  
      checkState(references.get(0).isDeclaration());
      BasicBlock initBlock = init.getBasicBlock();
      for (int i = 1; i < size; i++) {
        if (!initBlock.provablyExecutesBefore(references.get(i).getBasicBlock())) {
          return false;
        }
      }
  
      return true;
    }
docs: Determines if the variable for this reference collection is "well-defined." A variable is
  well-defined if we can prove at compile-time that it's assigned a value before it's used.
  
  <p>Notice that if this function returns false, this doesn't imply that the variable is used
  before it's assigned. It just means that we don't have enough information to make a definitive
  judgment.