Change a big rational .

Prototype
   int BigRat.change( brat , num1 , num2 )

Parameters
   num1
      String or Big Number or Float or nil representing the numerator.
   num2
      String or Big Number or Float or nil representing the denominator.

Remarks
   If num1 is a String or a Big Number or float, the numerator of bratis a representation if num1.
   If num1 is non nil and num2 is nil then brat is a rational with unity denominator.
   If num1 and num2 are nil, brat is a representation of number zero.

Example
   
require( "BigRat.lua" )

a = BigRat.new( "36846877697" , "3745856476754" )
BigRat.change( a , "358" , "694" )
print( a )

BigRat.change( a , "295 )
print( a )

BigRat.change( a )
print( a )
   
Output
   
358/694
295
0