;;;; -*-  Mode: LISP; Syntax: Common-Lisp; Base: 10                    -*-
;;;; ---------------------------------------------------------------------
;;;; File name: units.lsp
;;;;    System: FIRE
;;;;    Author: Praveen Paritosh
;;;;   Created: September 22, 2002 12:25:23
;;;;   Purpose: Handling Units in FIRE
;;;; ---------------------------------------------------------------------
;;;;  Modified: Friday, January 23, 2004 at 17:44:05 by paritosh
;;;; ---------------------------------------------------------------------

;;; The idea is to provide support for doing computation involving quantities
;;; which have associated SI units. This works on top of Gordon Novak's 
;;; implementation of unit handling. 
;;; The issues are -- 
;;; 1. Compatibility with Cyc's representation of units. 
;;; 2. Figuring out how exactly would one want to use this. 
;;; 3. Adding to Novak's collection of units those that are not present there 
;;;    (like BillionDollars). 
;;; 4. How to handle Cyc's ScalarInterval notion of quantity?
;;;
;;; The key idea is, that for every quantity, there is one right unit (SI,
;;; plus our own conventions). 
;;;
;;; Cyc's representation of a quantity is 
;;;   (UnitOfMeasureFn ?NUM) or (UnitOfMeasureFn ?MIN ?MAX)
;;; where UnitOfMeasureFn is a member of the collection UnitOfMeasure, and
;;; the above stands for a ScalarPointValue or a ScalarInterval respectively.
;;;  (standardUnit UnitOfSpeed MetersPerSecond)
;;;  (unitMultiplicationFactor MilesPerHour MetersPerSecond 2.2369)
;;; The Cyc representation of units is not compositional. So, for example, 
;;; it would have been more meaningful to say (/ Meter Second). Therefore
;;; for each of the derived units, we will add this knowledge to the KB.  
;;; 
;;; We have three kinds of things -- 
;;; 1. Dimension -- For example, Distance
;;; 2. Unit -- For example, Meter
;;; 3. Quantity -- An instantiation of a specific value on a dimension 
;;;      e.g., (Meter 4.0) or (Meter 4 10), or (
;;; The following kind of operations are supported -- 
;;;
;;; (convert <quantity> <unit>)
;;; (convert-to-standard-units <quantity>)