What is a null pdl ?
null is a special token for ’empty piddle’. A null pdl can be used to flag to a PDL function that it should create an appropriately sized and typed piddle. Null piddles can be used in places where a PDL function expects an output or temporary argument. Output and temporary arguments are flagged in the signature of a PDL function with the [o] and [t] qualifiers (see next question if you don’t know what the signature of a PDL function is).
