code: public ToStringBuilder appendToString(final String toString) {
          if (toString != null) {
              style.appendToString(buffer, toString);
          }
          return this;
      }
docs: <p>Append the <code>toString</code> from another object.</p>
  
  <p>This method is useful where a class delegates most of the implementation of
  its properties to another class. You can then call <code>toString()</code> on
  the other class and pass the result into this method.</p>
  
  <pre>
  private AnotherObject delegate;
  private String fieldInThisClass;
  
  public String toString() {
  return new ToStringBuilder(this).
  appendToString(delegate.toString()).
  append(fieldInThisClass).
  toString();
  }</pre>
  
  <p>This method assumes that the other object uses the same <code>ToStringStyle</code>
  as this one.</p>
  
  <p>If the <code>toString</code> is <code>null</code>, no change is made.</p>
  
  @param toString  the result of <code>toString()</code> on another object
  @return this
  @since 2.0