Implement #subst for part
Implement #to_f and improve integral_spec
Assert big refactoring to Set class
Assert refactoring to Set class
Refactoring set_spec
Improve #subst
Big improve for #subst
Remove training space
Merge pull request #25 from gogotanaka/implement#subst

Implement#subst
Merge pull request #27 from gogotanaka/improve_integral_calc

Improve integral calc