I want to define a non-terminal node in GP that can have different number of
children. For example, in a case it has two children, and in another case 5
children. However, it must have at least one child.

I tried to define a constrain with no size, but it does not work.

#No size is set
gp.nc.1 = ec.gp.GPNodeConstraints
gp.nc.1.name = undefinedChildren
gp.nc.1.returns = nil

Please let me know how can I define a node capable of having different
number of children.