1. A method for structural arithmetic processing, including: a) storing data words in several registers, each data word having a mark part and an information part, said mark part including a mark indicating if the register in question being in use or not, b) said data words being arranged in lists, storing each of said lists in a predetermined number of said registers, said mark part of each of said words in said lists stored in said registers being marked in use indicating that one of said lists has at least a part stored in a register, said list having a part stored in said register including a list instruction denoting which kind of list it is, c) arranging said lists stored in said registers as a tree of lists, of which one of the lists is a root list, where a relation between said lists is determined from an arrangement of said lists in said registers, d) controlling said registers and making use of said list instructions belonging to lists stored in said registers to rearrange said lists among said registers and to perform input/output operations of register content in accordance with said list instructions, wherein an information regarding what kind of structural arithmetic processing to perform can be derived from a type of said root list.