Undo accumulated position shifts from the data, windows and models.
EXAMPLE
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0>unshift
unshift: Removing accumulated position shifts.
0>

CONTEXT
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The 'shift' command can be used to incrementally shift the position of the
phase center of the data, and the associated positions of models and CLEAN
windows. The 'unshift' command puts the data back to the position that
they had when the observation was read into difmap.

ASSOCIATED COMMANDS
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shift - Shift the observation phase-center, CLEAN window and model positions.