Feature request: pointer differences in relocatable blocks

[davidgiven]

The code I'm trying to port does this a lot:

lda #message_end - message_start
message_end:
    dta c'This is a string'
message_start:

Unfortunately doesn't work inside relocation blocks, as the pointer difference fails with a 'Address relocation overload' error. However, while computing the pointer difference between labels in different blocks is impossible, it should be possible to compute this when the labes are in the same block, i.e. in the case above (and a number of other similar cases).

Would it be possible to add support for this? It should be just a matter of suppressing the error if the labels are in the same block and the arithmetic operation is a subtraction. I'm taking a look at the relevant code but am still trying to figure out the expression parser.

Thanks!