context function 3: op-if

op-if:
    description:
        op-if(condition, operator1, operator2) input-seq
        op-if condition evaluates to |yes> or |true> then apply operator1 to input-seq
        else apply operator2 to input-seq
        NB: the python version of if() is much uglier and clunkier.
        It is now implemented as value-if()

    examples:
        -- just define a couple of abstract operators:
        op1 (*) #=> |op1:> __ smerge[", "] |__self>
        op2 (*) #=> |op2:> __ smerge[", "] |__self>

        -- then apply it:
        op-if(|yes>, |op: op1>, |op: op2>) ssplit |abc>
            |op1: a, b, c>

        op-if(|no>, |op: op1>, |op: op2>) ssplit |abc>
            |op2: a, b, c>

    see also:
        value-if