class Tilt::FinalizedMapping

cannot be modified.
Private internal class for finalized mappings, which are frozen and

def clone(freeze: false)

Returns receiver, since instances are always frozen. 
def clone(freeze: false)
  self
end

def dup

Returns receiver, since instances are always frozen. 
def dup
  self
end

def extensions_for(template_class)

be used for.
Returns an aarry of all extensions the template class will 
def extensions_for(template_class)
  res = []
  @template_map.each do |ext, klass|
    res << ext if template_class == klass
  end
  res.uniq!
  res
end

def initialize(template_map)

explicitly check for that.
be frozen, but this is an internal class, so it does not
Set the template map to use. The template map should already 
def initialize(template_map)
  @template_map = template_map
  freeze
end

def lookup(ext) 

def lookup(ext)
  @template_map[ext]
end

def registered?(ext)

Return whether the given file extension has been registered. 
def registered?(ext)
  @template_map.has_key?(ext.downcase)
end