Utpl implements C-style pre- and postfix increment and decrement operators.

Pre-increment or -decrement operations first mutate the value and then return
the resulting value while post-increment or -decrement operations first return
the initial value and then mutate the operand.

Since the decrement and increment operators mutate their operand, they
may only be applied to variables, not constant literal expressions.

If an undefined variable is incremented or decremented, its initial value
is assumed to be "0".

If a non-numeric value is incremented or decremented, it is converted to a
number first. If the value is not convertible, the result of the increment
or decrement operation is NaN.

-- Expect stdout --
Incrementing a not existing variable assumes "0" as initial value:

 - Postfix increment result: 0, value after: 1
 - Prefix increment result:  1, value after: 1
 - Postfix decrement result: 0, value after: -1
 - Prefix decrement result:  -1, value after: -1

Incrementing a non-numeric value will convert it to a number:

124
3.5
2
NaN
-- End --

-- Testcase --
Incrementing a not existing variable assumes "0" as initial value:

 - Postfix increment result: {{ "" + a++ }}, value after: {{ a }}
 - Prefix increment result:  {{ "" + ++b }}, value after: {{ b }}
 - Postfix decrement result: {{ "" + c-- }}, value after: {{ c }}
 - Prefix decrement result:  {{ "" + --d }}, value after: {{ d }}

Incrementing a non-numeric value will convert it to a number:

{%
	n = "123"; n++; print(n, "\n");
	n = "4.5"; n--; print(n, "\n");
	n = true; n++; print(n, "\n");
	n = { some: "object" }; n--; print(n, "\n");
%}
-- End --