Running the following in a repl will give the first set of non-terminal symbol
"D" in the following grammar. Calling the followSet function instead will
return the follow set of "D".
firstSet (Grammar [
Rule { lhs = (N"A"), rhs = [[N"B", N"C"], [N"B", N"C", N"D", T"w"]] },
Rule { lhs = (N"B"), rhs = [[T"y"], [Epsilon]] },
Rule { lhs = (N"C"), rhs = [[T"x"], [Epsilon]] },
Rule { lhs = (N"D"), rhs = [[T"z"]] }
]
) (N "D")