SIGNATURE ISeqZip[from1,from2,to]

-- zip infinite sequences

-- Parameter
SORT from1 from2 to

IMPORT ISeq[from1] ONLY iseq
       ISeq[from2] ONLY iseq
       ISeq[to]    ONLY iseq

FUN zip :(from1**from2->to)->iseq[from1]**iseq[from2]->iseq[to]
        -- zip (compose) (s1, s2)
        -- zip the sequences s1, s2 by compose
        -- informally:
        -- zip (compose) (<a1, ... >, <b1, ... >) 
        --		== <a1 compose b1, ... >

FUN unzip :(to->from1**from2)->iseq[to]->iseq[from1]**iseq[from2]
        -- unzip (split) (s)
        -- unzip a sequence s by split in two 
        -- informally:
        --   compose (split) == identity => 
        -- 	unzip (split) (< a1 compose b1, ... >)
        --	== (<a1, ... >, <b1, ...>)