Functions
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  All Dust-commands allow mathematical expressions instead of
  plain numbers, other commands require expressions containing
  special variables:

   1. Special parameters required by the FUNC-commands:
     X0 - initial x-coordinate
     Y0 - initial y-coordinate
     Z0 - initial z-coordinate
     T0 - parameter
   Example: You want to plot the 2D-Function sin(x^2+y^2):
    -load a plane (dimensions -50..-50,-50..50,0..0)
    -now enter "func(2,"30*sin(x0*x0/30+y0*y0/30)",0,z)"
   This uses the x- and y-coordinates to modify the"
   z-coordinate of the plane."

   2.Constants
     'pi', 'ee' (2.71..)

   3.Operators
     '+','-', ':-(', '/', '^'

   4.Functions
      'jump','entier','int','abs','sqr','sqrt',
      'exp','ln','log','log10','log2','tentox','twotox',
      'sin','arcsin','cos','arccos','tan','arctan',
      'sinh','cosh','tanh','artanh', 'degtorad','radtodeg',
      'rnd','fac','ceil','floor','round');

  NOTES: 1. All angles in degrees.