compound ket fn: has-prefix

has-prefix:
    description:
        has-prefix["str"] ket
        returns |yes> if the ket has the given prefix

    examples:
        has-prefix["not "] |not sitting at the beach>
            |yes>

        has-prefix["not "] |sitting at the beach>
            |no>

    see also:
        has-suffix, remove-suffix, remove-prefix