fix: recognize unary operators

src/nodes/from_str.rs

@@ -78,9 +78,13 @@ impl FromStr for Nodes {
                 } else if c == ')' {
                     nodes.push(Node::ParenthesisEnd);
                 } else {
-                    nodes.push(Node::Operator(
+                    if matches!(nodes.last(), Some(Node::Operator(_)) | None) {
-                        c.try_into().map_err(|()| Error::InvalidOperator(c))?,
+                        state = ParsingState::ReadingNumber(i);
-                    ));
+                    } else {
+                        nodes.push(Node::Operator(
+                            c.try_into().map_err(|()| Error::InvalidOperator(c))?,
+                        ));
+                    }
                 }
             }
         }