class ComplexConfig::KeySource

def initialize(pathname: nil, env_var: nil, var: nil, master_key_pathname: nil)
  settings = [ pathname, env_var, var, master_key_pathname ].compact.size
  if settings > 1
    raise ArgumentError, 'only one setting at most possible'
  end
  pathname and pathname = pathname.to_s
  master_key_pathname and master_key_pathname = master_key_pathname.to_s
  @pathname, @env_var, @var, @master_key_pathname =
    pathname, env_var, var, master_key_pathname
end