Multiply two big rationals.

Prototype
   int BigRat.mul( brat1 , brat2 , brat3 )

Parameters
   brat1 and brat2
      Numbers to be multiplied.
   brat3
      Returns the multiplication of brat1 and brat2.

Remarks
   brat1, brat2, brat3 must be created before function call

Example
   
require( "BigRat.lua" )

a = BigRat.new( "639" )
b = BigRat.new( "563" )
result = BigRat.new( )
BigRat.mul( a , b , result )
print( a .. " * " .. b .. " = " .. result )

a = BigRat.new( "-543" )
b = BigRat.new( "1256" )
BigRat.mul( a , b , result )
print( a .. " * " .. b .. " = " .. result )

--Try to add non valid parameters
ret = BigRat.mul( a , d , result )
print( ret ) ;
   
Output
   
639 * 563 = 359757
-543 * 1256 = 682008
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