filter now works for unknown variables

bugs.txt

-When using d = prodigy.Dist("1") and then checking whether d.filterLess("s<10") results in a variable not known error. 

src/Dist.cpp

@@ -361,9 +361,13 @@ namespace prodigy {
         ex eExp = parse(e);
         EXPECTS(eExp.info(info_flags::nonnegint), "given expression must be non-neg int");
         int order = ex_to<numeric>(eExp).to_int();
-        EXPECTS(isKnownAsVar(x), "given variable is not known");
-        // todo one might accept unknown vars as well
-        const auto &xExp = _vars.at(x);
+        ex xExp;
+        if(!isKnownAsVar(x)) {
+            xExp = symbol {x};
+            Dist::registerAsVar(ex_to<symbol>(xExp));
+        } else {
+            xExp = _vars.at(x);
+        }
         ex series = _gf.series(xExp, order);
         return Dist{series_to_poly(series)};
     }

test/TestDist.cpp

@@ -55,3 +55,10 @@ TEST(Dist, filter) {
     ASSERT_EQ(dist.filterLess("x", "3").mass(), GiNaC::numeric(7,8));
     ASSERT_EQ(dist.filterLeq("x", "2").mass(), GiNaC::numeric(7,8));
 }
+
+TEST(Dist, filterUnknownVar) {
+    Dist::resetSymbolCache();
+    Dist dist {"1"};
+    ASSERT_TRUE(dist.isTrivial() == troolean::YES);
+    ASSERT_EQ(dist.filterLess("x", "1").mass(), GiNaC::numeric(1));
+}