Data type prefixes

+$a            # $a as a number
~$a            # $a as a string
?$a            # $a as a boolean

+@a            # number of elements in @a
~@a            # @a as a string (separated by spaces)
?@a            # true if @a not empty

+|             # numeric or
~|             # string bitwise-or
?|             # boolean or