# Just another scratch pad¶

## Notes on [Burstall1977]¶

Laws. Well expressed as

*known properties*about the objects. Associativity, commutativity, etc..`i + j`

— Does not commute on strings, but it does associate.`A * M`

— Commutes if A is a scalar and M a matrix, but does not commute if both are matrices. It associates for scalars and matrices alike being mixed.

In xotl.ql, this kind of information can be only provided by

`isinstance()`

inside a query expression, or by the object model (known only to the translator).