private void verifyType(TensorType givenType) {
        if (requiredType.isPresent() && ! givenType.isAssignableTo(requiredType.get()))
            throw new IllegalArgumentException("Variable '" + name + "' must be compatible with " +
                                               requiredType.get() + " but was " + givenType);
    }
A variable tensor which must be compatible with the given type
vespajlib/src/main/java/com/yahoo/tensor/evaluation/VariableTensor.java