We propose a simple problem generator that can generate new
objective functions. The problem generator starts with a string
of length 1 containing the symbol "@", takes as input a number of
grammatical rules of the form
where the string may contain the symbol "@" again, and applies these rules recursively at random to the resulting string. It stops after a prespecified number of recursions. Any unresolved occurences of the recursion symbol @ are then replaced by a variable name , (), where i is chosen randomly and the number of unknowns n has to be prespecified by the user.