code: private IndexableConstraint getClassFieldReaderForHashedAlpha(final AlphaNode alphaNode) throws IllegalArgumentException {
          final AlphaNodeFieldConstraint fieldConstraint = alphaNode.getConstraint();
  
          if (!(fieldConstraint instanceof IndexableConstraint)) {
              throw new IllegalArgumentException("Only support IndexableConstraint hashed AlphaNodes, not " + fieldConstraint.getClass());
          }
          // we need to get the first alpha in the map to get the attribute name that be use for the prefix of the
          // generated variable name
          return (IndexableConstraint) fieldConstraint;
      }
docs: Returns the {@link ClassFieldReader} for the hashed AlphaNode. The AlphaNode's constraint has to be a
  MvelConstraint. This is the only type of hashed alpha currently supported.
  
  @param alphaNode hashed alpha to get reader for
  @return ClassFieldReader
  @throws IllegalArgumentException thrown if the AlphaNode's {@link org.kie.spi.AlphaNodeFieldConstraint} is not a
  {@link MvelConstraint}.