anchor.diff:  
   (in-package #:math)
   
  -;;; %% Angular Measure
  +;; % Class for Dimensionless Measurement Domains
  +
  +(defclass dimensionless-measurement-domain (measurement-domain)
  +  ;; FIXME: For formulas resulting in dimensionless measurements, it
  +  ;; may be possible to define a method to provide 'advice' for the
  +  ;; measurement being sought (?)
  +  ;;
  +  ;; Perhaps it may simply be accepted that the semantics of a
  +  ;; dimensionless measurement must be provided in documentation
  +  ;; seperate to any programmed expression of the measurement
  +  ())
  +
  +;;; % Angular Measure
   
   (defclass plane-angle (measurement-class)
     ()
  -  (:metaclass measurement-domain)
  +  (:metaclass dimensionless-measurement-domain)
     ;; FIXME: #I18N
     (:print-name . "angular measure")
     (:print-label . "angular measure")
  
positive.code: (register-measurement-domain (find-class 'plane-angle))
  
  (defclass radian (measurement)
    ;; FIXME: Define as COMPOUND-MEASUREMENT-CLASS with units m m^-1
    ;; FIXME: Define corresponding STERADIAN class in measurement domain
    ;; SOLID-ANGLE, units m^2 m^-2
    ()
    (:metaclass plane-angle)
    (:print-name . "radian")
  
negative.code: ;; (scalar-magnitude (convert-measurement (make-measurement (rational pi) :|rad|) :|deg|))
  ;; => 180
  
  ;;; % Dimensionless Measure (Generic)
  
  (defclass dimensionless-measure (measurement-class)
    ;; FIXME: Consider undefining this class. It should be possible to
  