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>

Welcome to the world of abusing the shit out of Ant. My first full time job was developing Ant in unholy ways. Tens of thousands of lines of Ant at least, doing significant logic. If-then-else, for loops, math, procedures, date-time math.