Datatype for ML pretty printing (cf. mlsource/MLCompiler/Pretty.sml in Poly/ML 5.3).
;;; Options for Proof General
;; Examples for sensible settings:
;(custom-set-variables '(isar-eta-contract nil))
;(custom-set-faces
; '(proof-locked-face
; ((((type x) (class color) (background light)) (:background "lightsteelblue2")))))