SymPy .subs() bug fixed via dummy variables

Fix substitution bug in dt systems, pull request corrects bug in all places.

Thanks Zinoex!

Author: Kiguli

src/functions/dt_DS.py

@@ -113,7 +113,9 @@ def dt_DS(b_degree, dim, L_initial, U_initial, L_unsafe, U_unsafe, L_space, U_sp
     # ========================= Sub Difference Equations =========================
 
     # substitute the result of the difference equations
-    Barrier_f = Barrier.subs([(x[i], f[i]) for i in range(len(x))])
+    y = [sp.Dummy(f'y{i}') for i in range(len(x))]
+    Barrier_f = Barrier.subs([(x[i], y[i]) for i in range(len(x))])
+    Barrier_f = Barrier_f.subs([(y[i], f[i]) for i in range(len(y))])
 
     # ========================= Constraints =========================
     try:

src/functions/dt_SS.py

@@ -167,7 +167,9 @@ def dt_SS(b_degree, dim, L_initial, U_initial, L_unsafe, U_unsafe, L_space, U_sp
         return {"error": "Gamma, Lambda, or c value definition issues","b_degree":b_degree}
 
     # ========================= Sub Difference Equations =========================
-    BB = Barrier.subs([(x[i], f[i]) for i in range(len(x))])
+    y = [sp.Dummy(f'y{i}') for i in range(len(x))]
+    BB = Barrier.subs([(x[i], y[i]) for i in range(len(x))])
+    BB = BB.subs([(y[i], f[i]) for i in range(len(y))])
     BB = sp.expand(BB)
 
     for i in range(len(varsigma)):