1. A system for calculating the look ahead probabilities at the nodes in a language model look ahead tree, wherein the words of the vocabulary of the language are located at the leaves of the tree, said system comprising a processor adapted to: to obtain a first low order language model look ahead tree having a language first low order language model probability assigned to the words of the vocabulary and the language model look ahead probabilities calculated for nodes in said first low order language model look ahead tree; to determine if the language model probability of one or more words of said vocabulary can be calculated using a higher order language model and updating said words with the higher order language model; and to update the look ahead probability at the nodes of the first low order language model look ahead tree which are affected by the words where the language model has been updated, wherein the first low order language model look ahead tree is one order lower than the higher order language model.