class Rack::Builder

def generate_map(default_app, mapping)

map block in this instance.
Generate a URLMap instance by generating new Rack applications for each 
def generate_map(default_app, mapping)
  mapped = default_app ? { '/' => default_app } : {}
  mapping.each { |r, b| mapped[r] = self.class.new(default_app, &b).to_app }
  URLMap.new(mapped)
end