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Removed leftover of previous proof.
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milessabin committed Apr 18, 2018
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21 changes: 6 additions & 15 deletions _sips/sips/2017-11-20-byname-implicits.md
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Expand Up @@ -691,21 +691,12 @@ of types with complexity less than or equal to _c(T<sub>ps</sub>)_ is finite and
exhausted. This time, however, the sequence cannot be extended, because there are no more distinct
covering sets available to be introduced to avoid dominating an earlier element of the sequence ∎.

To show that all paths in the tree of implicit expansions are finite we must decompose them into
definitional subpaths as in the previous proof. All nodes are either byname or strict, and so each of
these definitional subpaths consist of zero or more byname nodes each separated by zero or more strict
nodes.

Call the sequence consisting of only the byname nodes of a definitional subpath, in path order, its
_byname subpath_. And call the set of (possibly empty) sequences of strict nodes separating each
byname node its _strict subpaths_. By construction, the byname subpath is non-dominating and so by
the lemma above must be finite. Each of the strict subpaths is also non-dominating by construction and
hence also finite. Consequently each of the definitional subpaths are the concatenation of a finite
number of finite byname nodes separated by a finite sequence of strict nodes, which must in turn be
finite.

Finally, as in the previous proof we rely on **P3** to show that there are only a finite number of
these finite definitional subpaths and hence that their interleaving must also be finite ∎.
Finally, as in the previous proof each path in the tree consists of nodes labelled with some element
of _D_ and so can be decomposed into an interleaving of definitional subpaths with respect to each of
those definitions. These definitional subpaths are non-dominating and hence, by the earlier lemma,
finite. **P3** asserts that there are only a finite number of these finite paths, so we know that
their interleaving must also be finite ∎.


#### Motivating example for the covering set based divergence critera

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