Fix LaTeX of free algebras (and repr_lincomb)

[kcrisman]

sage: K.< alpha,b > = FreeAlgebra(SR)
sage: latex(alpha*b)
\alphab

Someone we are not using repr_lincomb correctly, and in fact that function itself looks pretty hackishly defined. I couldn't even find a very good workaround.

See ​the first report at ask.sagemath.

[ppurka]

sage/misc/latex.py

@@ -2503 +2503 @@
         '\\alpha'
     """
     if a in common_varnames:
-        return "\\" + a
+        return "\\" + a + "{}"
     elif len(a) == 1:
         return a
     elif is_fname is True:

This fixes the latex, but the raw latex doesn't look good any more.

sage: K.< alpha,b > = FreeAlgebra(SR)      
sage: latex(alpha*b)                       
\alpha{}b

[ppurka]

In fact, what will break if we simply add a space after every such comman variable name?

sage/misc/latex.py

@@ -2503 +2503 @@
         '\\alpha'
     """
     if a in common_varnames:
-        return "\\" + a
+        return "\\" + a + " "
     elif len(a) == 1:
         return a
     elif is_fname is True:

Visually, and in latex, this is probably the simplest fix.

Edit: Ok. Visually, this breaks subscripts and superscripts although the latex is fine.