;	CALC CALCULATES THE VALUE OF A FULLY PARENTHESIZED EXPRESSION
;	IN INFIX. AN EXPRESSION IS 
	
;	1) AN ATOM WHICH IS EVALUATED
	
;	2) A LIST OF A SINGLE ELEMENT. THE SINGLE ELEMENT IS EVALUATED
	
;	3) A LIST OF 3 ELEMENTS. THE MIDDLE ELEMENT MUST BE AN OPERATOR
;	   +, -, * OR /. THE FIRST AND THIRD ELEMENT ARE EXPRESSIONS
	
(DEFINE CALC
,  (LAMBDA (EXP)
,  ,  (COND
,  ,  ,  ((ATOM EXP) (EVAL EXP))
,  ,  ,  ((NULL (CDR EXP))  (EVAL (CAR EXP)))
,  ,  ,  ((EQ (CADR EXP) '+)
,  ,  ,  ,  (PLUS (CALC (CAR EXP)) (CALC (CADDR EXP)))
,  ,  ,  ,,,)
,  ,  ,  ((EQ (CADR EXP) '*)
,  ,  ,  ,  (TIMES (CALC (CAR EXP)) (CALC (CADDR EXP)))
,  ,  ,  ,,,)
,  ,  ,  ((EQ (CADR EXP) '-)
,  ,  ,  ,  (DIFFERENCE (CALC (CAR EXP)) (CALC (CADDR EXP)))
,  ,  ,  ,,,)
,  ,  ,  ((EQ (CADR EXP) '/)
,  ,  ,  ,  (QUOTIENT (CALC (CAR EXP)) (CALC (CADDR EXP)))
,  ,  ,  ,,,)
,  ,  ,,,)
,  ,,,)
,,,)
	
	
	
;	PAREN TAKES A LIST OF THE FORM
	
;		(A + B * C - D)
	
;	AND FULLY PARENETHIZES IT. * AND / HAVE EQUAL PRIORITY WHICH IS
;	GREATER THAN + AND - (ALSO EQUAL)
	
(DEFINE PAREN 
,  (LAMBDA (SENT)
,  ,  (PROG (P)
,  ,  ,  (COND
,  ,  ,  ,  ((ATOM SENT)  (RETURN SENT))
,  ,  ,  ,  ((SETQ P (PREFIX SENT '+ '-))
,  ,  ,  ,  ,  (RETURN (LIST (PAREN (CADR P)) (CAR P) (PAREN (CADDR P))))
,  ,  ,  ,  ,,,)
,  ,  ,  ,  ((SETQ P (PREFIX SENT '* '/))
,  ,  ,  ,  ,  (RETURN (LIST (PAREN (CADR P)) (CAR P) (PAREN (CADDR P))))
,  ,  ,  ,  ,,,)
,  ,  ,  ,,,)
,  ,  ,  (RETURN SENT)
,  ,  ,,,)
,  ,,,)
,,,)
	
	
	
;	PREFIX TAKES AN EXPRESSION AN A SET OF TWO OPERATORS. IT FINDS
;	THE FIRST OCCURANCE OF EITHER OPERATOR AND REBUILDS THE EXPRESSION
;	INTO A 3 ELEMENT LIST. THE OPERATOR FOUND IS THE FIRST ELEMENT.
;	ALL THAT PRECEDED THE OPERATOR BECOMES THE 2ND ELEMENT. ALL THAT
;	FOLLOWED THE OPERATOR BECOMES THE THIRD ELEMENT
	
(DEFINE PREFIX
,  (LAMBDA (EXPRS T1 T2)
,  ,  (PROG (P F)
,  ,  ,  (SETQ P EXPRS)
,  ,  ,  (REPEAT
,  ,  ,  ,  UNTIL (NULL P)
,  ,  ,  ,  UNTIL (EQ (CAR P) T1)
,  ,  ,  ,  UNTIL (EQ (CAR P) T2)
,  ,  ,  ,  (SETQ F (CONS (CAR P) F))
,  ,  ,  ,  (SETQ P (CDR P))
,  ,  ,  ,,,)
,  ,  ,  (COND
,  ,  ,  ,  (P (RETURN (LIST (CAR P) (REVERSE F) (CDR P))))
,  ,  ,  ,,,)
,  ,  ,,,)
,  ,,,)
,,,)