Update SPEC.md

SPEC.md

@@ -4,12 +4,11 @@ This book is the specification for the Walnut configuration language that
 speifies syntax and semantics. Parser and interpreter implementations must
 follow this specification.
 
-Note that I used [TOML v1.0.0](https://toml.io/en/v1.0.0) as a sample to write
-this specification. I'm grateful to the authors and contributors.
-
 ## The terms
 - "Whitespace" means tab (`U+0009`) or space (`U+0020`).
 - "Newline" means line feed (`U+000A`) or carriage return (`U+000D`).
+- "Parenthesis" means left and right of round brackets (`()`), curly brackets
+ (`{}`), or square brackts (`[]`).
 
 ## Table of Contents
 - File Format
@@ -46,10 +45,23 @@ this specification. I'm grateful to the authors and contributors.
 - The appropriate MIME type for Walnut files is `application/walnut`.
 
 ## Line
+Line is the string between two [newline](#the-term) characters (or the
+beginning of a file), expcept newlines escaped by `\`, newlines surrounded by
+[parentheses](#the-terms) or newlines in [strings](#string). The former two
+cases are treated as a [whitespace](#the-term), but in the latter case, the
+behavior is decided by the kind of string. See also:
+[Expression/String](#string)
+
+[Whitespaces](#the-terms) in the beginning or the end of a line and a
+[comment](#comment) in the middle, or the end of a line will be ignored (= is
+equivalent to empty text). Therefore, a comment in the middle of a line will
+be interpreted as a whitespace, and one in the end of a line will be ignored.
+
 ### Comment
-Comment is an string starts with hash symbol (`#`, `U+0023`) outside a string,
-and ends with line feed (`U+000A`) or carriage return (`U+000D`). Line feeds
-and carriage returns are not permitted in comments. Comments are allowed in the end of [lines](#line) (before newline character).
+Comment is a text starts with hash symbol (`#`, `U+0023`) outside a
+[string](#string), (inside [parentheses](#the-term) are allowed) and
+continues until the appearance of a [newline](#the-term) character (not
+included). Newline characters are not permitted in comments.
 
 ```
 # This is an comment