'stack_overflow' with some kinds of recursive types

[amiller]

This example causes stack overflow:

 type tree = Tip
 | Bin of (tree * int * tree) authtype
 | UnBin of (tree * int * tree) 

This workaround avoids the problem:

 type node = (tree * int * tree)
 and tree = Tip
 | Bin of node authtype
 | UnBin of node