Array of arrays with filtering functions. Each function takes four parameters. The fisrt parameter is the type of filter. This could either of: var, smp, series, meta, x, y, z, data, network or guess (if not known). The second parameter is the key that usually correspond to a sample or variable factor name in the x or z object of the data. The third parameter is the operator to use which is one of: >, >=, <, <=, between, exact, like or not like and the four parameter is an array with the value or values that are needed for the corresponding evaluation. Here are some examples: [ x, class, exact, [ A ] ], [ guess, dose, between, [ 0.5, 2 ] ]