sequence function: and

    description:
        and(ket, ket)
        the and function
        NB: it evaluates both sequences before being passed to the 'and' function
        NB: it currently doesn't handle sequences all that well
      
    examples:
        and(|yes>, |yes>)
            |yes>
        
        and(|yes>, |no>)
            |no>
    
    see also:
      if, or, xor