better syn tree display

src/main.rs

@@ -10,7 +10,7 @@ mod parse;
 mod symbols;
 
 fn write_expr(expr: &Expr, writer: &mut impl Write, prefix: &str, is_last: bool) -> io::Result<()> {
-    let branch = if is_last { "+--" } else { "|--" };
+    let branch = if is_last { "└──" } else { "├──" };
 
     match expr {
         Expr::Int { value, .. } => writeln!(writer, "{}{}<{}>", prefix, branch, value),
@@ -22,7 +22,7 @@ fn write_expr(expr: &Expr, writer: &mut impl Write, prefix: &str, is_last: bool)
             let new_prefix = if is_last {
                 format!("{}    ", prefix)
             } else {
-                format!("{}|   ", prefix)
+                format!("{}│   ", prefix)
             };
 
             write_expr(lhs, writer, &new_prefix, false)?;