Files in this directory are data for Go's API checker ("go tool api", in src/cmd/api).

Each file is a list of API features, one per line.

go1.txt (and similarly named files) are frozen once a version has been
shipped. Each file adds new lines but does not remove any.

except.txt lists features that may disappear without breaking true
compatibility.

next.txt is the only file intended to be mutated. It's a list of
features that may be added to the next version. It only affects
warning output from the go api tool.
