Add two big rationals .

Prototype
   int BigRat.add( brat1 , bnum2 , bnum3 )

Parameters
   brat1 and bnum2
      Numbers to be added.
   brat3
      Returns the sum of brat1 and bnum2.

Remarks
   brat1 , bnum2 and bnum3 must be created before function call

Example
   
require( "BigRat.lua" )

a = BigRat.new( "639" )
b = BigRat.new( "563" )
result = BigRat.new( )
BigRat.add( a , b , result )
print( a .. " + " .. b .. " = " .. result )

a = BigRat.new( "-543" )
b = BigRat.new( "1256" )
BigRat.add( a , b , result )
print( a .. " + " .. b .. " = " .. result )

--Try to add non valid parameters
ret = BigRat.add( a , d , result )
print( ret ) ;
   
Output
   
639 + 563 = 1202
-543 + 1256 = 713
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