Remove underscore syntax matching any symbol

Underscore syntax:

For instance, a tree `(expr (id 'a') (add '+') (_))` matches both source codes `a + b * c` and `a - b / c`.

This feature is helpful because it allows you to emphasize the main points of the test by ignoring nodes of no interest.
However, we will remove the feature for the time being to reconsider the grammar.

3 files changed, 8 insertions, 59 deletions

diff --git a/README.md b/README.md
index 9c8d649..af7d693 100644
--- a/README.md
+++ b/README.md
@@ -132,11 +132,11 @@ a / b * 100
 ---
 (expr
 	(expr
-		(expr (id))
-		(div)
-		(expr (id)))
-	(mul)
-	(expr (int)))
+		(expr (id 'a'))
+		(div '/')
+		(expr (id 'b')))
+	(mul '*')
+	(expr (int '100')))
 ```
 
 The test case consists of a description, an input text, and a syntax tree you expect. Each part is separated by the delimiter `---`. The syntax tree is represented by the syntax like an [S-expression](https://en.wikipedia.org/wiki/S-expression).
@@ -150,8 +150,6 @@ Passed test.txt
 
 When you specify a directory as the 2nd argument of `vartan test` command, it will run all test cases in the directory.
 
-The underscore `_` allows you to match any symbols. Thus `(expr (expr (id)) (_) (expr (id)))` matches `a + b`, `a - b`, and so on.
-
 ### 5. Generate a parser
 
 Using `vartan-go` command, you can generate a source code of a parser to recognize your grammar.
diff --git a/spec/test/parser.go b/spec/test/parser.go
index e9c557d..aa5d022 100644
--- a/spec/test/parser.go
+++ b/spec/test/parser.go
@@ -99,8 +99,7 @@ func DiffTree(expected, actual *Tree) []*TreeDiff {
 	if expected == nil && actual == nil {
 		return nil
 	}
-	// _ matches any symbols.
-	if expected.Kind != "_" && actual.Kind != expected.Kind {
+	if actual.Kind != expected.Kind {
 		msg := fmt.Sprintf("unexpected kind: expected '%v' but got '%v'", expected.Kind, actual.Kind)
 		return []*TreeDiff{
 			newTreeDiff(expected, actual, msg),
diff --git a/spec/test/parser_test.go b/spec/test/parser_test.go
index cbf96c0..a12c858 100644
--- a/spec/test/parser_test.go
+++ b/spec/test/parser_test.go
@@ -75,26 +75,6 @@ func TestDiffTree(t *testing.T) {
 			),
 		},
 		{
-			t1: NewNonTerminalTree("_"),
-			t2: NewNonTerminalTree("a"),
-		},
-		{
-			t1: NewNonTerminalTree("a",
-				NewNonTerminalTree("_"),
-			),
-			t2: NewNonTerminalTree("a",
-				NewNonTerminalTree("b"),
-			),
-		},
-		{
-			t1: NewNonTerminalTree("_",
-				NewNonTerminalTree("b"),
-			),
-			t2: NewNonTerminalTree("a",
-				NewNonTerminalTree("b"),
-			),
-		},
-		{
 			t1:        NewNonTerminalTree("a"),
 			t2:        NewNonTerminalTree("b"),
 			different: true,
@@ -159,35 +139,6 @@ func TestDiffTree(t *testing.T) {
 			),
 			different: true,
 		},
-		{
-			t1: NewNonTerminalTree("a",
-				NewNonTerminalTree("_"),
-				NewNonTerminalTree("c"),
-			),
-			t2: NewNonTerminalTree("a",
-				NewNonTerminalTree("b"),
-				NewNonTerminalTree("x"),
-			),
-			different: true,
-		},
-		{
-			t1: NewNonTerminalTree("_"),
-			t2: NewNonTerminalTree("a",
-				NewNonTerminalTree("b"),
-			),
-			different: true,
-		},
-		{
-			t1: NewNonTerminalTree("a",
-				NewNonTerminalTree("_"),
-			),
-			t2: NewNonTerminalTree("a",
-				NewNonTerminalTree("b",
-					NewNonTerminalTree("c"),
-				),
-			),
-			different: true,
-		},
 	}
 	for i, tt := range tests {
 		t.Run(fmt.Sprintf("#%v", i), func(t *testing.T) {
@@ -398,12 +349,13 @@ foo x
 `,
 			parseErr: true,
 		},
+		// The error node cannot have another node.
 		{
 			src: `test
 ----
 foo x
 ----
-(foo (error (_ (x 'x'))))
+(foo (error (a)))
 `,
 			parseErr: true,
 		},