requirements - Type Error in Alloy specification -

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in requirements engineering (2007) article, "requirement progression in problem frames", there worked example on traffic lights problem have transcribed alloy editor. unfortunately, following error when testing code.

starting solver...

a type error has occurred: must set or relation. instead, has following possible type(s): {primitiveboolean}

the error triggered following predicate:

pred lightunitbreadcrumb [] {     t: time |          ngobserve [t] <=>              odd [ngpulse [t]] ,         sgobserve [t] <=>              odd [sgpulse [t]] }

referencing ngpulse predicate below:

sig ngp, sgp, nrp, srp in time {} pred ngpulse [t: time] {t in ngp} pred sgpulse [t: time] {t in sgp} pred nrpulse [t: time] {t in nrp} pred srpulse [t: time] {t in srp}

exactly. explain on page 137 of book why boolean not type in alloy.


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