  When using existing ACL2 datatype frameworks, many theorems require type
hypotheses. These hypotheses slow down the theorem prover, are tedious to
write, and are easy to forget. We describe a principled approach to types that
provides strong type safety and execution efficiency while avoiding type
hypotheses, and we present a library that automates this approach. Using this
approach, types help you catch programming errors and then get out of the way
of theorem proving.
