1. In history matching the results of computer implemented simulation with reservoir field production data from the reservoir to conform the reservoir simulation with the field production data, a computer implemented method to modify input data regarding the reservoir to more satisfactorily match the reservoir to the field production data based on specified modify instructions regarding simulator input data about the reservoir, the computer implemented method comprising the steps of: converting the modify instructions into a computer recognizable token; parsing the computer recognizable token according to a grammar to form a modify data structure; determining in the computer if the modify data structure identifies a reservoir variable, and if so, finding a pointer in computer memory to the variable; and determining in the computer if the modify data structure identifies an operator, and if so, performing the operation according to the identified operator data to modify the simulator input data.