Theorem stoic3

Stoic logic Thema 3. Statement T3 of [Bobzien] p. 116-117 discusses Stoic logic Thema 3. "When from two (assemblies) a third follows, and from the one that follows (i.e., the third) together with another, external assumption, another follows, then that other follows from the first two and the externally co-assumed one. (Simp. Cael. 237.2-4)" (Contributed by David A. Wheeler, 17-Feb-2019.)

Hypotheses
stoic3.1
\( \vdash \varphi \wedge \psi \rightarrow \chi \)
stoic3.2
\( \vdash \chi \wedge \theta \rightarrow \tau \)
Conclusion
\( \vdash \varphi \wedge \psi \wedge \theta \rightarrow \tau \)
Proof
l0
stoic3.1
\( \vdash \varphi \wedge \psi \rightarrow \chi \)
stoic3.2
\( \vdash \chi \wedge \theta \rightarrow \tau \)
sylan l0 _
3impa _