struct Input
{
  float3 worldNormal;
  // this macro in Input struct & call to WorldNormalVector required if you want to get world
  // space one from the o.Normal that you might just have written into
  INTERNAL_DATA
};
void surf(Input IN, inout SurfaceOutput o)
{
  o.Normal = /* sample the normal map etc */
  half3 worldNormal = WorldNormalVector(IN, o.Normal);
  // there!
}