code: private static SystemInputDef asSystemInputDef( JsonValue json)
      {
      try
        {
        SystemInputDef systemInput = null;
        if( json != null && json.getValueType() == OBJECT)
          {
          systemInput = SystemInputJson.asSystemInputDef( (JsonObject) json);
          }
  
        return systemInput;
        }
      catch( Exception e)
        {
        throw new ProjectException( "Error reading input definition", e);
        }
      }
docs: Returns the system input definition represented by the given JSON value.