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Required values analysis
------------------------
This analysis checks for each operation in a Curry program whether
the arguments must have a particular shape in order to
compute some value.
For instance, the negation operation `not` defined by
not True = False
not False = True
requires the argument value `False` in order to compute the result
`True` and it requires the argument `True` to compute the result
`False`. This property is expressed by the following abstract type:
not : ({True} -> {False}) | ({False} -> {True})
Hence, each abstract type is a set of constructors which represents
all expressions rooted by one of the constructors in this set.
Moreover, the abstract type `any` denotes any expression.
The empty list denotes an impossible required type, i.e.,
an argument which is required but for which no applicable value exists.
For instance, the operation
f x = solve (x && not x)
has the required value typing
f: ({} -> True)
A detailed description of this analysis and its application can be found in the
[LOPSTR'15 paper](http://www.informatik.uni-kiel.de/~mh/papers/LOPSTR15.html).
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