A scope is a formatting container which can contains line of text as well as other nested scopes

In other word a scope define an indentation section.

empty_scope () returns a brand new scope

line scope s adds s to scope

empty_line scope adds an empty line to scope

sub_scope scope f adds a sub scope and apply f to it.

print scope returns the formatted scops with a 2 character indentation for each scope.

Write to output