16. A method of calculating language model 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 method comprising: assigning a language model probability to each of the words of the vocabulary using a first low order language model; calculating the language look ahead probabilities for all nodes in said tree using said first language model; determining 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 probabilities; and updating the look ahead probability at only the nodes which are affected by the words where the language model has been updated.
25. A non-transitory computer readable medium storing program instructions for controlling a computer to carry out the method of claim 16 .