The 
                  citation element represents an author's acknowledgment of the original author and/or work of a directly or indirectly borrowed idea,
                  quote or other resource. Citations typically occur in conjunction with epigraphs, quotes, illustrations, charts and diagrams.
                  
               
               The 
                  Block variant is an adaptation for use where the citation is offset from the quoted material.
                  
               
               The optional 
                  
                     ref
                      attribute on the 
                  citation element is used to establish an explicit association between the 
                  citation and the passage or resource it references; the placement of the 
                  citation does not, by default, imply an association with any element in the document. Some elements do provide a mechanism for implied
                  relationships, however (see the 
                  
                     quote
                      element, for example). When adding citations, refer to the documentation for the element the citation is being attached to
                  for more information. 
                  
               
               A 
                  citation can also be linked to the work it cites by including a child 
                  
                     ref
                      element. A 
                  
                     ref
                      attribute can be attached to the nested 
                  ref element to reference a work in the current document's 
                  
                     bibliography
                     , for example. To reference other resources, including resources external to the current document, the 
                  
                     xlink:href
                      attribute must be used instead.
                  
               
             
            Usage Example
<quote>
    <p>By his return he was critical of the Bible as history, and wondered why all 
    religions should not be equally valid.</p>
    <citation>Darwin 1958, pp. 85–96</citation>
</quote>
    Allowed parents
            address, 
               annotation, 
               m:annotation-xml, 
               aside (block variant), 
               aside (toc variant), 
               backcover, 
               block, 
               caption, 
               description, 
               entry (bibliography variant), 
               entry (glossary variant), 
               flaps, 
               frontcover, 
               item, 
               d:longdesc, 
               note (block variant), 
               note (phrase variant), 
               object, 
               sel:otherwise, 
               quote, 
               section, 
               d:simplifiedLanguageDescription, 
               d:summary, 
               td, 
               th, 
               d:tour and 
               sel:when
               
            
            Allowed children
            This element may contain text.
            
            This element may contain the following children: abbr, 
               address, 
               annoref, 
               annotation, 
               ssml:break, 
               char, 
               citation, 
               code, 
               d, 
               definition, 
               emph (text variant), 
               emph (phrase variant), 
               expansion, 
               xforms:input, 
               rend:linebreak, 
               ln, 
               m:math, 
               name, 
               note, 
               noteref, 
               num, 
               object (text variant), 
               object (phrase variant), 
               pagebreak, 
               ssml:phoneme (text variant), 
               ssml:phoneme (phrase variant), 
               ssml:prosody (text variant), 
               ssml:prosody (phrase variant), 
               quote, 
               xforms:range, 
               ref, 
               its:ruby, 
               s, 
               ssml:say-as (text variant), 
               ssml:say-as (phrase variant), 
               xforms:secret, 
               sel:select, 
               xforms:select, 
               xforms:select1, 
               span (text variant), 
               span (phrase variant), 
               sub, 
               ssml:sub (text variant), 
               ssml:sub (phrase variant), 
               sup, 
               term, 
               xforms:textarea, 
               time, 
               ssml:token (text variant), 
               ssml:token (phrase variant) and 
               w
               
            
            Content model and additional requirements
            
               optional attributes: 
@about, 
                  
@by, 
                  
@class, 
                  
@content, 
                  
@continuation, 
                  
@datatype, 
                  
@desc, 
                  
@its:dir, 
                  
@its:translate, 
                  
@property, 
                  
@rel, 
                  
@resource, 
                  
@rev, 
                  
@role, 
                  
@sel:selid, 
                  
@ssml:onlangfailure, 
                  
@typeof, 
                  
@xml:base, 
                  
@xml:id, 
                  
@xml:lang and 
                  
@xml:space 
               optionally 
                  
                  a choice of
                     end of choice 
                  
                
               one or more of 
                  
                  a choice of either one of the 53 elements: 
abbr, 
address, 
annoref, 
annotation, 
char, 
citation, 
code, 
d, 
definition, 
emph, 
emph, 
expansion, 
its:ruby, 
ln, 
m:math, 
name, 
note, 
noteref, 
num, 
object, 
object, 
pagebreak, 
quote, 
ref, 
rend:linebreak, 
s, 
sel:select, 
span, 
span, 
ssml:break, 
ssml:break, 
ssml:phoneme, 
ssml:phoneme, 
ssml:prosody, 
ssml:prosody, 
ssml:say-as, 
ssml:say-as, 
ssml:sub, 
ssml:sub, 
ssml:token, 
ssml:token, 
sub, 
sup, 
term, 
time, 
w, 
w, 
xforms:input, 
xforms:range, 
xforms:secret, 
xforms:select, 
xforms:select1 or 
xforms:textarea 
                     or text
end of choice 
                  
                
               Note that in addition to restrictions presented in the content model  above, 
                  					use of this element must also respect the following requirements:
                  
                     - The 
                        
citation element must neither be empty nor contain only whitespace.
                        
                      
                     - The 
                        
ref attribute on the 
                        ref element must refer to an 
                        entry in a 
                        bibliography when nested inside a 
                        citation element.
                        
                      
                  
                  Such requirements take precedence over any conflicting statements in the
                     						content model or in the lists above of allowed children and parents.
                  
                
             
            Variants
            This element exists in 2 variants. The other variant is the phrase variant.
            Namespace
            http://www.daisy.org/ns/z3998/authoring/