mb event system 6 Sections EventSystems Doc
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Def @ix:T initially x = v(j)
Def == if eqof(IdDeq)(j,i) x : T initially x = v else  fi

is mentioned by

Thm* i:Id, T:Type, v:Tx:Id.
Thm* @ix:T
Thm* @ixinitially x = v 
Thm* realizes es.(vartype(i;xT)
Thm* realizes es.& (e:E. loc(e) = i  Id  first(e (x when e) = v  T)
[init-rule]

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mb event system 6 Sections EventSystems Doc