
  set (buffer) = expression..

  perform math operations on images.  there are several allowed
  operators.  the standard math functions are + - * /.  Exponentiation
  is performed with ^ (set c = a ^ b).  There are several "logic"
  operators which need some explanation.

  set c = a < b  -- c is 1 if a < b, 0 otherwise
  set c = a > b  -- c is 1 if a > b, 0 otherwise
  set c = m1 & m1 - c is 1 if m1 AND m2 true (non-zero is true)
  set c = m1 | m1 - c is 1 if m1 OR m2 true (non-zero is true)
  set c = a << b -- c is the minimum of a and b
  set c = a >> b -- c is maximum of a and b

  complex operations may be performed:

  set c = a*(a < b) + c*((a < c) | (c < d))

  unary operators also exist:

  exp(a) - e to the power of a
  ten(a) - 10 to the power of a
  ln(a) - log base e of a
  log(a) - log base 10 of a
  sqrt(a) - square root of a
  sin(a) - sin of a
  cos(a) - sin of a
  tan(a) - sin of a
  not(a) - logical inverse of a
  abs(a) - absolute value of a
  int(a) - integer value of a

  Examples:
  
  set a = b + 10  (add 10 to every pixel in b and put the answer in a)
  set a = a / b   (divide every pixel in a by the corresponding pixel
                     b and put the answer in a)
  set b = 10 / a  (divide 10 by every pixel in a and put the answer
                     in b)

