eclipse-aascw / aas-core-codegen / 35667968407

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Index and `len` a JSON object or array (#743)

An invariant may well index into a JSON-able object or array, and ask how
long one is.  The type inference has always allowed both, but the
transpilation did not follow.

Both operations are now decided on the inferred type of the collection, in
all the six backends.  A JSON-able *value* is refused for either: its shape
is known only at run time, so neither an index nor a length can be resolved
statically, and a boolean or a number is no container at all.

An index gives an *optional* JSON-able value, as the key might be missing
from the object and the position might lie outside the array.  The guards
which a meta-model already writes strip that optionality:

* ``key in mapping`` tells us that ``mapping[key]`` is there;
* ``len(array) > n``, and the other spellings of it, tell us how long
  the array is, and hence which of its positions are there, counted from
  its front as well as from its back.

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