private TensorType outputType(TensorType inputType) {
        var m = inputType.mappedSubtype();
        var i = inputType.indexedSubtype();
        var d = function.outputType(i);
        if (m.rank() < 1) {
            throw new IllegalArgumentException(
                    "filter_subspaces needs input with at least 1 mapped dimension, but got: " + inputType);
        }
        return inputType;
    }
The <i>filter_subspaces</i> tensor function selects some cells or subspaces in a mapped or mixed tensor

@author arnej
vespajlib/src/main/java/com/yahoo/tensor/functions/FilterSubspaces.java