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# Well-Definedness Checking

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Well-definedness errors can occur in B in the following circumstances:

• a division by 0: r=100/0
• modulo by 0, modulo involving negative numbers: r=100 mod -2
• exponentiation using a negative exponent: 2**(-2)
• application of a function outside of its domain: f={1|->2} & r=f(3)
• application of a relation which is not a function: f={1|->2,1|->3} & r=f(1)
• application of the card operator to infinite sets: card(NATURAL)
• application of the min or max operator to empty sets: x={} & r=max(x)
• application of the inter operator to empty sets: x={} & r=inter(x)
• wrong application of sequence operators
• taking the first, front, last, tail of an empty sequence: x=<> & r=first(x)
• using negative indexes for prefix or suffix sequence: x = s /|\ -1
• using too large indexes for suffix sequence: x= s \|/ (size(s)+1)
• ...

To ensure that your models only contain well-defined formulas, Rodin creates well-definedness proof obligations for Event-B and Atelier-B can create them for classical B. ProB will check for well-definedness during constraint solving, animation and model checking. However, by default this check is not very extensive. You can force ProB to look more aggressively for well-definedness issues in your formulas by setting the following preference to true:

• TRY_FIND_ABORT

However, even with this, the search is not guaranteed to be exhaustive. Also, ProB's constraint solving does not necessarily solve formulas from left to right. Hence, the following formulas will be considered well-defined (and on can argue that they are) by ProB:

• r=1/x & x/=0

Technically speaking, for disjunctions and implications ProB uses the L-system of well-definedness (i.e., for P => Q, P should be well-defined and if P is true then Q should also be well-defined).