private void checkReferenceArguments(ReferenceNode feature, int fromArg) {
        for (int i = fromArg; i < feature.getArguments().size(); ++i) {
            ExpressionNode arg = feature.getArguments().expressions().get(i);
            if ( ! (arg instanceof ReferenceNode)) {
                throw new IllegalArgumentException("Invalid argument " + i + " to " + feature.getName() + ": " +
                        "the argument must be a reference. Got " + arg.toString());
            }
        }
    }
Transforms a feature of the form

    tokenAttentionMask(128, a, b, ...)

to an expression that generates a tensor that has values 1 for all
arguments (including CLS and SEP tokens) and 0 for the rest of the
sequence.

Concretely, transforms to a tensor generation expression:

    tensor(d0[1],d1[128])(if(d1 < 1 + length_a + 1 + length_b + 1 + ..., 1, 0))
config-model/src/main/java/com/yahoo/schema/expressiontransforms/TokenTransformer.java