The Symbolic Regression example in the docs here has several sections where the description does not match the code. For example, the first code block ends in

pset.addPrimitive(math.cos, 1)

But the description says

"On the last line, we declare an MetaEphemeral constant. This is a special terminal type, which does not have a fixed value. When the program appends an ephemeral constant terminal to a tree, the function it contains is executed, and its result is inserted as a constant terminal. In this case, those constant terminals can take the values -1, 0 or 1."