code: public DifferentialFunction getVariableOutputFunction(String variableName) {
          Preconditions.checkState(variables.containsKey(variableName), "No variable with name \"%s\" found in graph", variableName);
          if(variables.get(variableName).getOutputOfOp() == null)
              return null;
          return ops.get(variables.get(variableName).getOutputOfOp()).getOp();
      }
docs: Get the differential function (if any) that this variable is the output for
  
  @param variableName Name of the variable
  @return The differential function that this variable is an output of, or null if it is not the output of a function