Two of Fractint's faithful users went bonkers when we introduced the "formula" type, and came up with all kinds of variations on escape-time fractals using trig functions. We decided to put them in as regular types, but there were just too many! So we defined the types with variable functions and let you, the overwhelmed user, specify what the functions should be! Thus Scott Taylor's "z = sin(z) + z^2" formula type is now the "fn+fn" regular type, and EITHER function can be one of sin, cos, tan, cotan, sinh, cosh, tanh, cotanh, exp, log, sqr, recip, ident,. zero, one, conj, flip, cosxx, asin, asinh, acos, acosh, atan, atanh, sqrt, abs, or cabs.
Plus we give you 4 parameters to set, the complex coefficients of the two functions! Thus the innocent-looking "fn+fn" type is really 729 different types in disguise, not counting the damage done by the parameters!
Lee informs us that you should not judge fractals by their "outer" appearance. For example, the images produced by z = sin(z) + z^2 and z = sin(z) - z^2 look very similar, but are different when you zoom in.