Consider the rules that define directly or undirectly the goal f and let X be a list of a(f) distinct variables. Since f(X) is free w.r.t. X by hypothesis, it is possible to rewrite the variables in the above rules so that they are a subset of X.
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Article I details the main obligations of states parties, who undertake: Never under any circumstances: (a) To develop, produce, otherwise acquire, stockpile or retain chemical weapons, or transfer, directly or undirectly, chemical weapons to anyone; […]
Source: wiktionary