sequence function: for

    description:
        for(sp1, operators)
        for(sp1, sp2, operators)
        for(sp1, sp2, sp3, operators)
        the for loop function
        nested loops over the given superpositions, applying operators specified in 'operators'

    examples:
        -- just some abstract tests:
        merge-elts (*,*) #=> |_self1> _ |: > _ |_self2>
        merge-dots (*,*) #=> |_self1> _ | .. > _ |_self2>
        for(|0> + |1>, |2> + |3>, |op: merge-elts>)
            |0: 2> + |0: 3> + |1: 2> + |1: 3>
              
        for(|0> + |1>, |2> + |3>, |op: merge-elts> + |op: merge-dots>)
            |0: 2> + |0 .. 2> + |0: 3> + |0 .. 3> + |1: 2> + |1 .. 2> + |1: 3> + |1 .. 3>

        bracket (*) #=> |[> _ smerge[", "] |_self1> _ |]>            
        print (*) #=> print bracket |_self1>
        print (*,*) #=> print bracket(|_self1> . |_self2>)
        print (*,*,*) #=> print bracket(|_self1> . |_self2> . |_self3>)
        
        for(|a> + |b> + |c>, |op: print>)
            [a]
            [b]
            [c]

        for(split |a b c>, split |a b c>, |op: print>)
            [a, a]
            [a, b]
            [a, c]
            [b, a]
            [b, b]
            [b, c]
            [c, a]
            [c, b]
            [c, c]        

        for(split |a b>, split |a b>, split |a b>, |op: print>)
            [a, a, a]
            [a, a, b]
            [a, b, a]
            [a, b, b]
            [b, a, a]
            [b, a, b]
            [b, b, a]
            [b, b, b]
        
    see also:
        sfor, smap