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 The first two functions of this menu allow to set
the limits of Feynman 'x' variable. The input must 
satisfy the condition
               0 <= x-Min < xMax <=1
 The third function allows the user to define the 
number of points for the argument (not greater than 
150) to be used for the plot.
 The fourth menu function allows the user to define
the QCD scale, which is a hidden parameter of the
partonic distribution functions.
  The last menu function 'Display plot' draws the
plot on the screen.

The minimal and the maximal values of the argument
'x' are x_Min + dx and x_Max-dx, correspondingly, 
where dx=(x_Max-x_Min)/2/Npoints. The endpoints x=0 
and x=1 do not appear on the plot.