In the following example, the <add> element represents a call to the XPRESS function named add.
<add> <ref>counter</ref> <i>10</i> </add>
This function is passed two arguments:
first argument– value is determined by calling a function named ref. The argument to the ref function is a literal string that is assumed to be the name of a variable. The value returned by the ref function is the current value of the variable counter.
second argument -- value is determined by calling a function named i. The argument to the i function is a literal string that is an integer. The value that the i function returns is the integer 10.
The value returned by the add function will then be the result of adding the integer 10 to the current value of the variable counter. Every function call returns a value for the next operation to use. For example, if the ref call returns the value of the counter, then the <i> call returns the integer 10, then the <add> call returns the addition of the two calls.