learn rules: +=>

 +=> :
        op |ket> +=> sequence
        add the sequence on the right hand side to the existing definition of op|ket>
        if initially op|ket> is not defined, then just set it to the sequence
        useful for piece by piece construction of a superposition
        if the right hand side is a general sequence, it will be compiled to a sequence before being added to op|ket>

        -- learn Sam's friends one ket at a time:
        friends |Sam> +=> |Tom>
        friends |Sam> +=> |Matt>
        friends |Sam> +=> |Emma>
        friends |Sam> +=> |Mary>
        friends |Sam> +=> |Luke>

        -- now see the full list:
        sa: friends |Sam>
            |Tom> + |Matt> + |Emma> + |Mary> + |Luke>

    see also:
        add-learn, => , .=> , #=> , !=>