Accumulator Parameter Syntax
----------------------------

Author: Tom Goodale
Date: May 2002

Basically these are parameters who's value is built up from values of
other parameters which the base parameter doesn't know about , number
4 in the 'things for parameters' thread.

Previously we called these 'arithmetically-set' but think
'accumulator' is a better word (Can anyone think of an even better
one?)

Ok, so on to syntax

base parameters would be defined like

REAL foo "The foo parameter" accumulator(<expression>)
{
  (1:500 :: "Anything greater than 1 and less than or equal to 500" 
} 72

and parameters which modify this one's value would be like

REAL foobar "The foo parameter" accumulator-base(foo)
{
  22:65 :: "Sensible number" 
} 42

<expression> is an arbitrary arithmetical expression involving the old
value of the parameter,refered to as 'x', and the parameter which is
modifying it, refered to as 'y'.  E.g.  x+y, x+(1/y) x*y, x+y^2,
x/y,...  The expression should commute when applied twice, i.e. for
expression L(x,y), we should have

L(c,L(a,b)) = L(b,L(a,c))

(This allows people to use x/y as an expression which would end up as
a/b/c = a/c/b .)

Both the base parameter and the modifying parameters need to be of the
same type.