gh-39523: multi polynomial element/repr
    
This is an identical replacement for #39502 which had to be closed.
    
URL: #39523
Reported by: Martin Rubey
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/rings/polynomial/multi_polynomial_element.py

@@ -467,6 +467,8 @@ def _repr_(self):
             sage: repr(-I*y - x^2)  # indirect doctest
             '-x^2 + (-I)*y'
         """
+        if self.is_gen():
+            return self.parent().variable_names()[self.degrees().nonzero_positions()[0]]
         try:
             key = self.parent().term_order().sortkey
         except AttributeError:

src/sage/rings/semirings/tropical_mpolynomial.py

@@ -653,16 +653,28 @@ def _repr_(self):
         r"""
         Return a string representation of ``self``.
 
+        Note that ``x`` equals ``0*x``, which is different from
+        ``1*x``.  Therefore, we represent monomials always together
+        with their coefficients, to avoid confusion.
+
         EXAMPLES::
 
             sage: T = TropicalSemiring(QQ)
             sage: R.<x,y> = PolynomialRing(T)
             sage: x + R(-1)*y + R(-3)
             0*x + (-1)*y + (-3)
+
         """
         if not self.monomial_coefficients():
             return str(self.parent().base().zero())
-        s = super()._repr_()
+        try:
+            key = self.parent().term_order().sortkey
+        except AttributeError:
+            key = None
+        atomic = self.parent().base_ring()._repr_option('element_is_atomic')
+        s = self.element().poly_repr(self.parent().variable_names(),
+                                     atomic_coefficients=atomic,
+                                     sortkey=key)
         if self.monomials()[-1].is_constant():
             if self.monomial_coefficient(self.parent()(0)) < 0:
                 s = s.replace(" - ", " + -")