+Focus is a prefix class used to set the keyboard focus on
    a specific GUI element. For example, it can be used to put the initial focus
    on a specific input field in a search dialog. Examples of this usage can be
    found in the demo app supplied with the PicoLisp distribution, in gui.l:
    
    (de choCuSu (Dst)
       (diaform '(Dst)
          (<grid> "--.-.-."
             ,"Number" (gui 'nr '(+Var +NumField) '*CuSuNr 10)
             ,"Name" (gui 'nm '(+Focus +Var +TextField) '*CuSuNm 30)
             ,"Phone" (gui 'tel '(+Var +TelField) '*CuSuTel 20)
             (searchButton '(init> (: home query)))
             ,"Zip" (gui 'plz '(+Var +TextField) '*CuSuPlz 10)
             ,"City" (gui 'ort '(+Var +TextField) '*CuSuOrt 30)
             ,"Mobile" (gui 'mob '(+Var +TelField) '*CuSuMob 20)
             (resetButton '(nr nm tel plz ort mob query)) )
             .
             .
             .
      
      In the code above, +Focus is used to give initial focus to
      the "Name" input field.