context function 3: op-sfor

op-sfor:
    description:
        op-sfor(fn, seq)
        op-sfor(fn, seq1, seq2)
        op-sfor(fn, seq1, seq2, seq3)
        apply fn to each of the superpositions in the given sequences, using a Cartesian product

    examples:
        -- for loop over single parameter function:
        fn (*) #=> |fn(> _ smerge[", "] |__self> _ |)>
        op-sfor( |op: fn>, split (|abc> . |de> . |xyz>))
            |fn(a, b, c)> . |fn(d, e)> . |fn(x, y, z)>

    see also:
        op-for