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Previously the functions `is_prefix` and `plant` did not take the
situation of packed nodes into considerations. That was because I
only dealt with non-packed nodes in the past: the fragment to test for
prefixes and for planting did not intersect the packed nodes in the
forest, and the grammar is so simple that the fragments do not contain
packed nodes.
Then a test revealed this situation, so I have to fix this lack of
considerations now. This commit attempts to fix this issue.
From the newly added unit-tests, it seems that this fix works. :)
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Now the binding part is finished.
What remains is a bug encountered when planting a fragment to the
forest which intersects a packed node, which would lead to invalid
forests. This will also cause problem when planting a packed
fragment, but until now my testing grammars do not produce packed
fragments, so this problem is not encountered yet.
I am still figuring out efficient ways to solve this problem.
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There were two main issues in the previous version.
One is that there are lots of duplications of nodes when manipulating
the forest. This does not mean that labels repeat: by the use of the
data type this cannot happen. What happened is that there were cloned
nodes whose children are exactly equal. In this case there is no need
to clone that node in the first place. This is now fixed by checking
carefully before cloning, so that we do not clone unnecessary nodes.
The other issue, which is perhaps more important, is that there are
nodes which are not closed. This means that when there should be a
reuction of grammar rules, the forest does not mark the corresponding
node as already reduced. The incorrect forests thus caused is hard to
fix: I tried several different approaches to fix it afterwards, but
all to no avail. I also tried to record enough information to fix
these nodes during the manipulations. It turned out that recording
nodes is a dead end, as I cannot properly syncronize the information
in the forest and the information in the chain-rule machine. Any
inconsistencies will result in incorrect operations later on.
The approach I finally adapt is to perform every possible reduction at
each step. This might lead to some more nodes than what we need. But
those are technically expected to be there after all, and it is easy
to filter them out, so it is fine, from my point of view at the
moment.
Therefore, what remains is to filter those nodes out and connect it to
the holy Emacs. :D
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