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doc/ref/impl: fix order of arguments in δ partial feature function
The previous text, quite possibly copied verbatim from upstream
sources, defined δ as being Label x Q -> Q but in several places used
it as Q x Label -> Q. To me, this use is the more intuitive API.
- Updated the definition to match the use.
- Added a few extra words around paths to add clarity too.
Signed-off-by: Matthew Sackman <matthew@cue.works>
Change-Id: I8fad8938affb611032e3a39ff344fbf056cc2834
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197344
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
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doc/ref/impl.md
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doc/ref/impl.md
···
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> 1. `q0 ∈ Q`, is the root node,
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> 1. `υ: Q → T` is the total node typing function,
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> for a finite set of possible terms `T`.
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> 1. `δ: Label × Q → Q` is the partial feature function,
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> 1. `δ: Q × Label → Q` is the partial feature function,
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>
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> subject to the following conditions:
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>
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> 1. there is no node `q` or label `l` such that `δ(q, l) = q0` (root)
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> 2. for every node `q` in `Q` there is a path `π` (i.e. a sequence of
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> members of Label) such that `δ(q0, π) = q` (unique root, correctness)
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> 3. there is no node `q` or path `π` such that `δ(q, π) = q` (no cycles)
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> 3. there is no node `q` or non-empty path `π` such that `δ(q, π) = q` (no cycles)
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>
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> where `δ` is extended to be defined on paths as follows:
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>
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> 1. `δ(q, ϵ) = q`, where `ϵ` is the empty path
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> 2. `δ(q, l∙π) = δ(δ(l, q), π)`
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> 2. `δ(q, l∙π) = δ(δ(q, l), π)`
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>
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> The _substructures_ of a typed feature structure are the
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> typed feature structures rooted at each node in the structure.