function 3: value-if

value-if:
    description:
        value-if(condition, seq1, seq2) input-seq
        if condition evaluates to |yes> or |true> then return seq1, else return seq2
        input-seq is ignored

    examples:
        value-if( |yes>, |branch a>, |branch b>)
            |branch a>

        value-if( |no>, |branch a>, |branch b>)
            |branch b>

    see also:
        op-if