Assertions in storage/mutable.py provide assurance that the struct library performs correct size transforms

[zancas]

The assert statements at the top of this module are intended to provide assurance that the correct size is indicated by the arguments to the various struct methods used by the mutable file. A syntax error was preventing the intended function of these asserts. The attached patch corrects that syntax and provides the desired assurance.

In particular the relevant '>' format character is inserted in the assertion, and the statement is converted into a comparison with '=='.

[warner]

looks good! landed in d2e5de3b6913082f, with some additional improvements (emit the failing size) in 01b5124d0af5e6fe. thanks!