Typo in 1.6: f out of scope, should have been g

The variable f is bound on the right-hand side, and is instantiated with
an expression containing g. Thus, post-substitution, the expression
should contain g, and not f.
add 7fdadbf to errata
Merge pull request #374 from gelisam/master

Typo in 1.6, f instead of g when verifying that ac is well-typed.