Instead of adding all Sigs then rulling them out, only add in joins

Instead of marking every Sig as a candidate initially, only mark those
as candidates that are actually involved in a “field-access” dot-join.

There might still be false positives, or perhaps some false negatives,
but this should weed out some more false positives.

org.aminb.alloy.catalyst/src/main/java/org/aminb/alloy/catalyst/SSRVisitor.java

@@ -66,10 +66,10 @@ public class SSRVisitor extends VisitReturn {
         // `var in sig', discard candidate
         if (x.op == ExprBinary.Op.JOIN) {
             if (x.left instanceof ExprUnary && x.right instanceof ExprUnary)
-                if (!isFieldAccess(x)) {
+                if (isFieldAccess(x)) {
                     PrimSig s = getFirstPrimSig(x.left);
                     if (!builtins.contains(s))
-                        candidates.put(s, false);
+                        candidates.put(s, true);
                 }
         }
         else {
@@ -150,8 +150,16 @@ public class SSRVisitor extends VisitReturn {
                                 candidates.put((PrimSig)subsub, false);
                 }
             }
-            else if (s instanceof PrimSig)
-                System.out.println("no :( " + (PrimSig)s);
+            else if (s instanceof PrimSig) {
+                if (((PrimSig)s).getFieldDecls().size() > 1)
+                    if (!builtins.contains((PrimSig)s))
+                        candidates.put((PrimSig)s, false);
+            }
+            /**
+             * else: mostly `ExprVar's with empty type which we don't
+             * care about at all, or `ExprBinary's, which I think we
+             * handle well enough in their own visit(ExprBinary) call.
+             */
         }
         for (Decl d : x.decls)
             visitThis(d.expr);
@@ -205,8 +213,10 @@ public class SSRVisitor extends VisitReturn {
         // add the sig as a candidate if we haven't seen it before
         if (x instanceof PrimSig) {
             PrimSig px = (PrimSig)x;
+            /*
             if (!builtins.contains(px) && !candidates.containsKey(px))
                 candidates.put(px, true);
+            */
             if (px.isMeta != null)
                 candidates.put(px, false);
             if (px.isAbstract != null)