Feature request: rate update in interactive mode

[hmedina]

Previously, one could use $UPDATE to change a rule's rate via a perturbation. This feature has since been removed. Although it remains technically possibly to do this via a variable, that requires anticipating the need to change the rate, and defining it as a variable from the get-go. It would be useful, specially in interactive mode, to be able to change a rule's rate.

Given that rule-names point to a unique line of code, the only hurdle is in specifying which rate of the rule. I suggest the following for syntax:
$UPDATE [rule-name] ( [forward|reverse|unary] ) [new-value]
For example,
'foo' A(a),B(b) <-> A(a!1),B(b!1) @ 1{2},3
$UPDATE 'foo' (forward) 5