Deny moves without any equal sign

board-shared/src/expr.rs

@@ -80,7 +80,11 @@ pub fn is_valid_guess(board: &Board, positions: &[Position2d]) -> Result<bool, (
 
     let tokens = lexer(&tiles);
     let expressions = parser::parse(&tokens)?;
-    Ok(are_valid_expressions(&expressions))
+    if expressions.len() < 2 {
+        Ok(false)
+    } else {
+        Ok(are_valid_expressions(&expressions))
+    }
 }
 
 /// Convert an infix expression to a postfix expression.