The expressions should take the form of any real or complex valued function. The expressions e, pi, and i are reserved e = 2.7182818284590452354, pi = 3.1415926535897932385, and i is the imaginary unit. In complex plane mode the variable z is the dynamic variable and c is the constant or roaming constant. In real plane mode, x and y are the dynamic variables and c and d are the coordinates of the constant or roaming constant.
In addition, the variables a and b are reserved roaming constants. In complex plane mode both a and b take on the value of the current point in the plane and remain constant through the iteration of the point. Basically they work like another c value except if you are in Julia set mode then c is fixed and z is dynamic, so if you want the current point value in the formula just use either a or b. In real plane mode, a is the x-coordinate of the current point and b is the y-coordinate of the current point. Again these stay fixed through the orbit of the point.
Arithmetic operations use the standard characters (+, -, *, / and ^) with the standard order of operations. Juxtaposition is not supported. So for example, the expression 2z would cause a syntax error but 2*z would be syntactically correct. You may use standard log, exponential, trigonometric and hyperbolic functions. The set of built-in functions are listed below.