Specification of comments (#4)

Co-authored-by: Patrycja Balik <root@pbalik.com>

Author: ppolesiuk

src/ref/lexer.md

@@ -9,10 +9,95 @@ by the lexer, but they separate tokens, e.g., identifiers or literals.
 
 ## Comments
 
+Fram supports two kinds of comments: single-line and block comments. Comments
+are treated similarly to whitespace: they are ignored by the lexer, but they
+always separate tokens.
+
+Single-line comments start with the `#` character and end with a new line or
+the end of file.
+
+Block comments are introduced by the sequence `{#` followed by any, possibly
+empty, sequence *name* of valid identifier characters, operators, and
+`#` characters. Such a block comment is terminated by the first occurrence of
+*name* that is immediately followed by `#}`. More precisely, it can be
+described by the following grammar.
+```bnf
+block-comment-name  ::= { ident-char | op-char | "#" }
+block-comment-start ::= "{#" block-comment-name
+block-comment-end   ::= block-comment-name "#}"
+```
+Non-terminal symbols `ident-char` and `op-char` are defined later in this
+chapter. At the comment opening, the longest consecutive sequence described by
+`block-comment-name` is taken as the comment name. This name should be a suffix
+of the name provided at comment closing. Comments using the same name cannot
+be nested. This is not an issue in practice, since the programmer can always
+choose a unique name to accomplish nesting.
+
+By convention, single-line comments starting with `##` and block comments
+with a name starting with `#` are used as documentation comments, and can be
+recognized by some external tools.
+
+### Examples
+
+The following code contains some valid comments.
+````fram
+# This is a single-line comment.
+{# This is a block comment. #}
+{# Block comments
+  may span multiple lines.
+#}
+let id x = x # A single-line comment may appear at the end of a line.
+
+let n {# A block comment may span a part of a single line. #} = 42
+{#aaa
+Comments cannot be nested,
+{# but the programmer may choose the comment delimiters. #}
+aaa#}
+
+{#!a! Comment names may contain operators. !a!#}
+
+{#abc
+This comment is ended by `abc` immediately followed by `#}`,
+even if the closing sequence is preceded by other characters.
+zzabc#}
+
+let {#
+# This is not a single-line comment,
+# because comments are not nested.
+# This comment can be ended #} here = 13
+
+## This is a documentation comment.
+let foo x = x
+
+{## This is another documentation comment. ##}
+let bar = foo
+
+{###
+Documentation comments can contain some code
+```
+{## with another documentation comment (with a different name). #}}
+let some_code = 42
+```
+###}
+let baz = bar
+````
+
 ## Literals
 
+```bnf
+digit        ::= "0"..."9"
+lower-char   ::= "a"..."z" | "_"
+upper-char   ::= "A"..."Z"
+ident-char   ::= lower-char | upper-char | digit | "'"
+```
+
 ## Keywords
 
 ## Identifiers
 
 ## Operators
+
+```bnf
+op-char ::= "<" | ">" | "&" | "$" | "?" | "!" | "@" | "^" | "+" | "-"
+          | "~" | "*" | "%" | ";" | "," | "=" | "|" | ":" | "." | "/"
+```