How do you use %api.namespace in 2.7.7

[Arthur Schwarz]

Win 7
Cygwin
bison (GNU Bison) 2.7.1

I am using:
  %file-prefix "Slip.bison"
  %name-prefix "Slip"
  %output "Slip.Bison.cpp"
  %define api.namespace slip

I would like the generated file to be in the "slip" namespace. Bison elects to 
put it into the "Slip" namespace and generates the error message:
slip.y:103.10-22: error: %define variable 'api.namespace' is not used

How do I get the correct namespace and remove the error message (I can't seem 
to find the bison 2.7.1 manual so I can't trace this down).

thanks
art