@Article{tofte94:pshom,
  author = 	 {Mads Tofte},
  title = 	 {Principal Signatures for Higher-Order Modules},
  journal = 	 {Journal of Functional Programming},
  year = 	 1994,
  volume =	 4,
  number =	 3,
  pages =	 {285--335},
  month =	 {July}
}