code: @Override
      public void setData(FieldContent data) {
          // allow setting in field once only.
          // cannot have multiple sources for one @In !
          if (this.data != null) {
              throw new ComponentException("Attempt to set @In field twice: " + comp + "." + field.getName());
          }
          this.data = data;
      }
docs: called in 'in' access