I am new in ECJ. I'm going to use ECJ to find the solution of a second
degree equation, that is, x=(-b+sqrt(b^2-4ac))/(2a).
I've implemented as FunctionSet the following nodes: add, sub, sqrt,
square, mul, div, neg, a, b, c, 2 and 4.
I've followed the instructions described both in the tutorial4 in the
documentations provided with ECJ and the one in the ECJ manual.

What I need and donot find the way, is to force to appear in the result
tree the nodes: a, b, c, 2, 4, -, +, sqrt, square, div and mul. That is, to
specify in some manner that the nodes must appear at least once. Moreover,
I'd like to know if it is possible to set that one node must appear a
number of times in the solution.

I hope to have explained well.

Thank you very much in advance.