PhD Progress: Covering non-necessary terms

When covering an action from the state, it is likely that non-necessary terms (with regards to the action in question) will also be in the rule (such as on(X,Z), where Z isn’t part of the action). An example of such a rule is:
on(a,c) & cl(a) & on(b,d) & cl(b) -> move(a,b)

Note that ‘c’ and ‘d’ aren’t part of the action so when the action is inversely subsumed, these need to be put into variables of sorts. However, if they were given concrete named variables, they may cause problems when unifying later, as the variables would need to be checked if they can sumsume one-another.

A possible fix for this (though perhaps too rough), is to swap these variables with special terms ‘_’ (don’t care, again seen in FOXCS). These don’t care terms can be anything (within type constraints) and do not necessarily have to be inequal to one-another ‘_(1)’ can equal ‘_(2)’. However, on creation of the final rule for the action, these ‘_’ terms need to be concretely put into variable terms, though again without the inequals predicate bounding them from each other.

Currently this issue isn’t a big one, but this is mostly due to the small domain size and lack of background predicates being included in the observations.

The process of inversely substituting terms could be performed with Replacements, but I have a feeling it may proceed more smoothly by using writing the actions in string form and using the existing rule parser to create the rules. Strings hold advantage by allowing simple equality checks, omission of state terms and dealing with state terms, and special behaviour when parsing the rule (‘_’ term).

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