The Instruction operation "subtract" can be used to subtract numerical inputs. Each input is subtracted in turn from the preceding input.
The example below shows how a number of inputs will be arithmetically subtracted. Notice how the subtraction starts at the first input working sequentially to the last input, so we have (22 - 5) - 3 = 14.
<instruction operation="subtract">
<input value="22" />
<input value="5" />
<input value="3" />
</instruction>
14
In the example below we add the result of a number "20", to the result of a subtraction, arithmetically our equation would look like this, so we have 20 + (20 - 5) = 35.
<instruction xmlns="http://www.obyx.org">
<input>
<instruction operation="add">
<input value="20" />
<input>
<instruction operation="subtract">
<input value="20" />
<input value="5" />
</instruction>
</input>
</instruction>
</input>
</instruction>
35