class Less::Engine

Our monkeypatch keeps it around.
which is unpleasant for error reporting.
By default Less doesn’t preserve the filename of the file being parsed,
The entry point to Less.

def initialize_with_sass(obj, opts = {}) 

def initialize_with_sass(obj, opts = {})
  initialize_without_sass(obj, opts)
  @filename = obj.path if obj.is_a?(File)
end

def parse_with_sass 

def parse_with_sass
  parse_without_sass
rescue Sass::SyntaxError => e
  e.modify_backtrace(:filename => @filename)
  raise e
end