Add some playground
Improve error handling
Merge pull request #8 from kishikawakatsumi/error_handling

Error handling
Fix crash
Add allKeys()
Revert "Add allKeys()"

This reverts commit 7bcf2a52df784bae83ac59d04f4abba99d8d0061.
Revert "Revert "Add allKeys()""

This reverts commit 159a452508a0d436eb684ed0dc4134cd8a2fdc9c.
Merge pull request #9 from kishikawakatsumi/allkeys

Revert "Revert "Add allKeys()""
convienience initializer
clean up
Update README
refine
Merge pull request #10 from kishikawakatsumi/interface

convienience initializer
Merge pull request #11 from kishikawakatsumi/failable

clean up
Merge pull request #12 from kishikawakatsumi/readme

Update README
Update README.md
Merge pull request #13 from kishikawakatsumi/p

refine