StaticExpression and Expression should be mixed on same Implementation

[master-q]

Implementation is defined as following:

data Implementation a = Implement { pos            :: a
-- snip--
                                  , _iExpression   :: Either (StaticExpression a) (Expression a) -- ^ Expression (or static expression) holding the function body.
                                  }

But I found we can mix StaticExpression and Expression. See following:

https://github.com/githwxi/ATS-Postiats/blob/fa34c1617360a24dac56d0002484f3d9868e8bb7/doc/EXAMPLE/MISC/intsqrt_prf.dats#L75