/======\
F==!/=!\?\+#  | />-<-\
    |    \@\-@/@\===?/<#
    |      |    |
$+++/======|====/
    !    /=/       /+<<-\
    |    \!/======?\>>=?/<#  dup
    |      \<<+>+>-/
    !      !
    \======|====\
    |      |    |
    |  /===|==\ |
M==!\=!\?\#|  | |
         \@/-@/@/===?\<#
        ^       \>-<-/
        | ^  ^ ^ ^
        | |  | | subtract from n
        | |  | mutual recursion
        | |  recursion
        | n-1
        check for zero