Siles and opening them while retaining position

[zerothi]

Describe the feature Currently nested openings of files are cumbersome.

It would be great if the (ascii) siles allows to step arbitrarily in the files.

Something like:

with sile:
    A1 = sile.read_*()
    with sile:
         B1 = sile.read_*()
         B2 = sile.read_*()
    A2 = sile.read_*()
assert A1 == B1
assert A2 == B2

it would ease lots of the functionality around, and especially output files where other settings in the files determine whether we can find certain stuff in them. (settings etc.)