docs: def bound_symbols(self):
          """Set of bound SymPy symbols contained within the equation."""
          try:
              lhs_syms = self.lhs.bound_symbols
          except AttributeError:
              lhs_syms = set()
          try:
              rhs_syms = self.rhs.bound_symbols
          except AttributeError:
              rhs_syms = set()
          return lhs_syms | rhs_syms