fmt.spad line 322 [edit on github]
OperatorHandlers is an auxiliary domain. It is used holds functions for each operator of OutputForm, classified by the arity and the name of that operator.
handler(x, n, op) is only valid if knownHandler?(n, op) is true. It returns the respective handler stored in x.
knownHandler?(x, n, op) checks whether a handler for operator op is known. The number n denotes the number of arguments that the operator can take. It is OK if both knownOperator?(m, op) and knownOperator?(n, op) return true for different m and n.
new() creates an empty structure of handlers.
removeHandler!(n, op) removes any handler from an internal data structure (for the pair (n, op) such that afterwards knownHandler?(x, n, op) returns false.
setHandler!(x, n, op, hdl) puts hdl into the data structure x such that it can be queried afterwards.