Is there any schema language for XML that allows for specifying a constraint that there can be no cyclic references between elements.
As a toy example:
<animal name="A" eats="B">
<animal name="B" eats="C">
<animal name="C" eats="D">
would validate, but
<animal name="A" eats="B">
<animal name="B" eats="C">
<animal name="C" eats="D">
<animal name="D" eats="A">
would not validate.