code: public JBBPDslBuilder Custom(final String type, final String name, final String param) {
      final ItemCustom custom = new ItemCustom(type, name, this.byteOrder);
      custom.bitLenExpression = param == null ? null : assertExpressionChars(param);
      this.addItem(custom);
      return this;
    }
docs: Add named parametric custom variable.
  
  @param type  custom type, must not be null
  @param name  name of the field, can be null for anonymous
  @param param optional parameter for the field, can be null
  @return the builder instance, must not be null