It uses XML-like syntax:

<fun> <name>sum</name> <in> <int>foo</int> <int>bar=0</int> </in> <out><int>foo+bar</int></out> </fun>

<fun> <name>sum</name> <in> <int> <name>foo</name> </int> <int> <name>bar</name> <default_value> <int>0</int> </default_value> </int> </in> <out> <int> <calculation> <numerical_operation> <operator_plus> <operand> <var>foo</var> </operand> <operand> <var>bar</var> </operand> </operator_plus> </numerical_operation> </calculation> </int> </out> </fun>

How did you come up with such a ****ingly stupid syntax?!

It haunts me in my dreams