Multi-step diagrams: Deleting & changing object fields

[dwhatley19]

From this diagram:
http://web.mit.edu/dwhatley/www/snapdown/web/production.html#a%20-%3E%20(A)%0A---%0Aa%20add%20b%20-%3E%201

Currently there is no way to delete the b field, or change what b points to.

Proposed syntax for deletion:

a -> (A)
---
a add b -> 1
---
a remove b

For changing what b points to: currently unknown. Possible to reassign it with b -x> then b -> 2, for example, but this creates two arrows.