alloy - Adding an removing relation between state -


i have alloy model

module test sig b {}  sig { c: b, delta: c lone -> lone c }   pred operationa[disj x, x': a,   c1, c2: b]{ x'.delta = x.delta + (c1->c2) x'.delta = x'.delta - (c1->c2) x'.c = x.c }   run operationa 10 2

this not generate me instance. adding relation c1->c2 in x' state , removing again, reason not allow me that.

the alloy language declarative, meaning write specification of what has happen in end , not how make happen (which imperative approach). therefore, body of operationa predicate interpreted conjunction of 3 lines wrote there, , not sequence of operations performed.

more concretely, line x'.delta = x'.delta - (c1->c2) says content of x'.delta relation at same time equal x'.delta - c1->c2. on it's own, line satisfiable if , if x'.delta not contain tuple c1->c2 (because - operator set difference, , if try remove set that's not in there, result same set). however, previous line, x'.delta = x.delta + (c1->c2) says content of x'.delta must include c1->c2 (plus whatever in x.delta, 2 lines unsatisfiable.

if want model 3 steps of imperative algorithm, you'll have use 3 variable, e.g., x, x', , x''.


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