Print node description with tostring()

[Stéphane Veyret]

Currently, nodes are printing their description in case of check failure. That would be good to be able to access this description at will, for example, overriding the __tostring() method of the types metatable, so that tostring(my_type) would display something more interesting than table <xxxxx>.

Thank you.

[leaf]

good suggestion, will be available in next release