resolve issue #97, add support for name=value@ attribute syntax

- also change default attribute value from 1 to empty string in cli options

2 files changed, 20 insertions, 13 deletions

diff --git a/man/asciidoctor.1 b/man/asciidoctor.1
index 166e9907..83a2f08f 100644
--- a/man/asciidoctor.1
+++ b/man/asciidoctor.1
@@ -2,12 +2,12 @@
 .\"     Title: asciidoctor
 .\"    Author: [see the "AUTHORS" section]
 .\" Generator: DocBook XSL Stylesheets v1.76.1 <http://docbook.sf.net/>
-.\"      Date: 01/22/2013
+.\"      Date: 01/23/2013
 .\"    Manual: \ \&
 .\"    Source: \ \&
 .\"  Language: English
 .\"
-.TH "ASCIIDOCTOR" "1" "01/22/2013" "\ \&" "\ \&"
+.TH "ASCIIDOCTOR" "1" "01/23/2013" "\ \&" "\ \&"
 .\" -----------------------------------------------------------------
 .\" * Define some portability stuff
 .\" -----------------------------------------------------------------
@@ -66,19 +66,23 @@ from the default safe mode of
 \fB\-a, \-\-attribute\fR=\fIATTRIBUTE\fR
 .RS 4
 Define, override or delete a document attribute\&. Command\-line attributes take precedence over attributes defined in the source file\&.
-
+.sp
 \fIATTRIBUTE\fR
 is formatted as a key\-value pair, in the form
 \fINAME=VALUE\fR\&. Values containing spaces should be enclosed in double\-quote characters\&. Alternate acceptable forms are
 \fINAME\fR
 (the
 \fIVALUE\fR
-defaults to an empty string) and
+defaults to an empty string),
 \fINAME!\fR
-(delete the
+(deletes the
+\fINAME\fR
+attribute) and
+\fINAME=VALUE@\fR
+(does not override
 \fINAME\fR
-attribute)\&.
-
+attribute if already defined in the source file)\&.
+.sp
 This option may be specified more than once\&.
 .RE
 .PP
diff --git a/man/asciidoctor.ad b/man/asciidoctor.ad
index 1f4149ff..e4cff1a0 100644
--- a/man/asciidoctor.ad
+++ b/man/asciidoctor.ad
@@ -48,12 +48,15 @@ Document Settings
 
 *-a, --attribute*='ATTRIBUTE'::
     Define, override or delete a document attribute. Command-line attributes
-    take precedence over attributes defined in the source file. +
-    'ATTRIBUTE' is formatted as a key-value pair, in the form 'NAME=VALUE'.
-    Values containing spaces should be enclosed in double-quote characters.
-    Alternate acceptable forms are 'NAME' (the 'VALUE' defaults to an empty
-    string) and 'NAME!' (delete the 'NAME' attribute). +
-    This option may be specified more than once. 
+    take precedence over attributes defined in the source file.
++
+'ATTRIBUTE' is formatted as a key-value pair, in the form 'NAME=VALUE'. Values
+containing spaces should be enclosed in double-quote characters.  Alternate
+acceptable forms are 'NAME' (the 'VALUE' defaults to an empty string), 'NAME!'
+(deletes the 'NAME' attribute) and 'NAME=VALUE@' (does not override 'NAME'
+attribute if already defined in the source file).
++
+This option may be specified more than once. 
 
 *-b, --backend*='BACKEND'::
     Backend output file format: 'docbook45' or 'html5'.  You can also use the