code: public DiffBuilder append(final String fieldName,
              final DiffResult diffResult) {
          validateFieldNameNotNull(fieldName);
          Validate.isTrue(diffResult != null, "Diff result cannot be null");
          if (objectsTriviallyEqual) {
              return this;
          }
  
          for (final Diff<?> diff : diffResult.getDiffs()) {
              append(fieldName + "." + diff.getFieldName(),
                     diff.getLeft(), diff.getRight());
          }
  
          return this;
      }
docs: <p>
  Append diffs from another {@code DiffResult}.
  </p>
  
  <p>
  This method is useful if you want to compare properties which are
  themselves Diffable and would like to know which specific part of
  it is different.
  </p>
  
  <pre>
  public class Person implements Diffable&lt;Person&gt; {
  String name;
  Address address; // implements Diffable&lt;Address&gt;
  
  ...
  
  public DiffResult diff(Person obj) {
  return new DiffBuilder(this, obj, ToStringStyle.SHORT_PREFIX_STYLE)
  .append("name", this.name, obj.name)
  .append("address", this.address.diff(obj.address))
  .build();
  }
  }
  </pre>
  
  @param fieldName
  the field name
  @param diffResult
  the {@code DiffResult} to append
  @return this
  @throws IllegalArgumentException
  if field name is {@code null}
  @since 3.5