“…A partial explanation to the lack of such results was revealed recently when it was shown in a number of papers, how powerful or difficult language equations are. To mention a few examples, it is shown in [12] that even the question whether or not, for given finite sets A, B, C, D, E, F , the equation AB i C = DE i F holds for all i ≥ 0, is recursively undecidable, see also [14]; the maximal set commuting with a given finite set A need not be recursive, see [13]; or the fact that two given finite sets A and B are conjugate, i.e., that there exists a set Z such that AZ = ZB holds, is known to be decidable only in the case of bifix sets, see [4].…”