code: public String displayName(Value value) {
          if (value == null) {
              throw new IllegalArgumentException("value cannot be null");
          }
  
          ValueSetElement vse = this.values.get(value);
          if (vse != null) {
              return vse.getDisplayName();
          } else {
              return "";
          }
      }
docs: Returns a display name for the specified value, if one is defined.
  
  @param value a {@link Value}. Cannot be <code>null</code>.
  @return a {@link String}.