private static ExpressionFunction makeLengthFunction() {
        String func = "sum(map(input, f(x)(x > 0)))";
        String name = "__token_length";
        try (var r = new StringReader(func)) {
            return new ExpressionFunction(name, List.of("input"), new RankingExpression(name, r));
        }
        catch (com.yahoo.searchlib.rankingexpression.parser.ParseException e) {
            throw new IllegalStateException("unexpected", e);
        }
    }
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