NOT.2:5.4 - Resolve a movable label at the time of its use

Three physical cups have permanent numbers 1, 2 and 3, volumes 20, 10 and 0 ml, and capacity 50 ml each. Removable tags A, B and C initially mark cups 1, 2 and 3 respectively. An instruction H says to transfer 5 ml from A to B. Each letter refers to the cup bearing that tag when the transfer is performed.

Write H; swap-tags(B, C); H, where the swap moves only tags. The first H gives volumes (15, 15, 0) by permanent cup number. After the swap, B marks cup 3. The second H therefore gives (10, 15, 5). Both transfers have sufficient source volume and destination capacity.

The abbreviation H retains a lookup to perform, not cup numbers fixed when H was defined. If the intended instruction instead bound A and B permanently to the original cups, the same two transfers would give (10, 20, 0). The designer must choose the rule matching the work. Changing the tags is an action on the represented situation; renaming a letter in the notation while preserving its reference is a different change.