TDL := versus :<

[bond at cslab.kecl.ntt.co.jp]

Thanks Ulrich, Chris and Ann.

In summary (for the FAQ):

What is the difference between ":=" and ":<"?

The original definitions are that ":=" allows one to specify AVM
constraints, while ":<" only allows a single supertype.  Because of
this difference you can chain ":<"s, for example:

   a :< b :< c.

which is the same as:

  a :< b.
  b :< c.

or 

  a := b.
  b := c.

PET makes this distinction, and allows chaining.

LKB deprecates ":<" and interprets it in exactly the same way as ":=".