We study operations on fixpoint equation systems (FES) over arbitrary complete lattices. We investigate under which conditions these operations, such as substituting variables by their definition, and swapping the ordering of equations, preserve the solution of a FES. We provide rigorous, computer-checked proofs. Along the way, we list a number of known and new identities and inequalities on extremal fixpoints in complete lattices.Well-known instances of FES. Boolean Equation Systems (BES) arise as FES over the complete lattice ⊥ < ⊤, and were proposed in [And94, AV95] for solving the model checking and equivalence checking problems on finite labeled transition systems (LTS). BES received extensive study in [Mad97, MS03, GK04, Mat06].An equivalent notion to BES is two-player parity games [EJ91], see [Mad97] for a proof. Algorithms for solving parity games receive a lot of attention, since this is one of the few problems which is in NP and in co-NP, but not known to be in P. Recently, it has been shown that parity games (and thus BES) can be solved in quasi-polynomial time [CJK + 17]. This result has also been lifted to the general setting of FES on finite lattices [HS21, JMT22]. Other types of games can also be seen as an instance of FES, for example energy parity