--> ****************************************************************
-->                      Generic Sequences
--> ****************************************************************

--> ----------------------------------------------------------------
--> SEQ: sequences (i.e. associative list) with {id: emp}
--> ----------------------------------------------------------------
mod! SEQ (X :: TRIV) {
[Elt < Seq]
-- empty Sequence
op emp : -> Seq {constr} . 
op __ : Seq Seq -> Seq {constr assoc id: emp} .
}

--> ----------------------------------------------------------------
--> TRIV=: TRIV with _=_
--> ----------------------------------------------------------------
mod* TRIV= {
[Elt]
-- equality on Elt
pred _=e_ : Elt Elt {comm} .
eq (E:Elt =e E) = true .
cq [:nonexec]: E1:Elt = E2:Elt if (E1 =e E2) .
}

--> ----------------------------------------------------------------
--> SEQ= : SEQ with _=_
--> ----------------------------------------------------------------
mod! SEQ= (X :: TRIV=) {
pr(SEQ(X))
-- equality on Seq
pred _=s_ : Seq Seq {comm} .
eq (S:Seq =s S:Seq) = true .
cq [:nonexec]: S1:Seq = S2:Seq if (S1 =s S2) .
-- for the case if either side is emp and other side is non-emp
eq (emp =s (S1:Seq E:Elt S2:Seq)) = false .
-- for the case if both sides are non-emp
eq (E1:Elt S1:Seq =s E2:Elt S2:Seq) = ((E1 =e E2) and (S1 =s S2)) .
}

--> ----------------------------------------------------------------
--> SEQ=hd : SEQ= with hd_
--> ----------------------------------------------------------------
mod! SEQ=hd {
pr(SEQ=)
-- head (leftmost element) of a Seq
op hd_ : Seq -> Elt . 
eq hd (E:Elt S:Seq) = E .
}

--> ----------------------------------------------------------------
--> provide seq
provide seq

--> ****************************************************************
--> end of file
eof
--> ****************************************************************