RNN Shakespeare example giving error

Start by locating the RNN Shakespeare tutorial and the referenced rnn_model.R, rnn.R, and lstm.R files. Reproduce the reported check.data error from the shown mx.lstm call and verify whether the missing files or package references explain it. Done means the tutorial's example runs without the reported error and all referenced files are available or correctly documented.

This code in the tutorial leads to the error:

model <- mx.lstm(X.train, X.val, 
                 ctx=mx.gpu(),
                 num.round=num.round, 
                 update.period=update.period,
                 num.lstm.layer=num.lstm.layer, 
                 seq.len=seq.len,
                 num.hidden=num.hidden, 
                 num.embed=num.embed, 
                 num.label=vocab,
                 batch.size=batch.size, 
                 input.size=vocab,
                 initializer=mx.init.uniform(0.1), 
                 learning.rate=learning.rate,
                 wd=wd,
                 clip_gradient=clip_gradient)

Error: Error in check.data(train.data, batch.size, TRUE) : could not find function "check.data"
Is this function from an unspecific package?

Additionally, there are several missing files referenced in the tutorial:
rnn_model.R, rnn.R, lstm.R, etc

best,