--
> 
> Heres a BNF version of something close to SKIF, which is about the 
> simplest logical BNF I can think of. You tell me how to bundle-ize 
> it. The exact use of parens here is lisp style, but that's not 
> important. We could miss out the <term> clause and say 
> relationalAtomicSentence ::= (word+) if that would be easier, but it 
> would be a pity.
> 
> word ::= qname |URIreference |Variable
> <comment> analogous to identifiers in programming language; basic 
> lexical items used to 'name' things </comment>
sentence ::= (relAtomicSentence | propSentence |quantSentence)
word ::= (qname|URIreference|variable)
term ::= (word | {word term*} | [term term term])

as list:
term ::= (word | [word term list]|[word term|nil nil] | [term term term])
> relAtomicSentence ::= (term+)
> propSentence ::= (not sentence) | (and|or sentence*) | (=>|iff 
> sentence sentence)
> quantSentence ::= (forall|exist (variable+) sentence)
> document ::= sentence*
>
relAtomicSentence ::= [term {term*} nil]
propSentence ::= ([not sentence nil] | [(and|or) {sentence*} nil] | [(iff|implies) sentence sentence])
quantSentence ::= [(forall|exist) variable|{variable+} sentence]

relAtomicSentence ::= [term term list]|[term term|nil nil]
varList ::= [etc variable varList|nil]
quantSentence ::= [(forall|exist) variable|varList sentence]

Now suppose a 'RDF' schema language (as an extension to RELAXNG
http://relaxng.org and whose formal semantics 
could be written as an
extension of the formal semantics of RELAXNG). The 
purpose of this language
is to declare constraints on the values of the 
elements of triples. This is
in "tree regular expression" form and mirrors 
the form of the schema for the
RDF Surface Syntax http://www.openhealth.org/RDF/RDFSurfaceSyntax 
or
http://www.openhealth.org/RDF/RDF.rng
<define name="sentence">
    
<choice>
        <ref 
name="relAtomicSentence"/>
        
<ref name="propSentence"/>
        
<ref name="quantSentence"/>
    
</choice>
</define>
<define name="relAtomicSentence">
    
<statement>
        
<predicate>
            
<term/>
        
</predicate>
        
<subject>
            
<bundle/>
        
</subject>
        
<object>
            
<nil/>
        
<object>
    
</statement>
</define>
<define 
name="propSentence">
    
<choice>
        
<statement>
            
<predicate 
name="not"/>
            
<subject>
                 
<ref 
name="sentence"/>
            
</subject>
        
</statement>
        
<statement>
            
<choice>
               
<predicate 
name="and"/>
               
<predicate 
name="or"/>
            
</choice>
            
<subject>
                
<bundle/>
            
</subject/>
        
</statement>
        
<statement>
                
<choice>
                    
<predicate 
name="iff"/>
                    
<predicate 
name="implies"/>
                 
</choice>
                
<subject>
                    
<ref 
name="sentence"/>
                
</subject>
                
<object>
                    
<ref 
name="sentence"/>
                
</object>
        </statement></define><define 
name="quantSentence">
    
<statement>
          
<choice>
            
<predicate 
name="forall"/>
            
<predicate 
name="exists"/>
          
</choice>
        
<subject>
            
<oneOrMore>
                
<variable/>
            
</oneOrMore/>
        
</subject>
        
<object>
            
<ref name="sentence"/>
        
</object>
    
</statement>
</define>