xdi-graph               = *( xdi-statement / CRLF )
xdi-statement           = contextual-statement / literal-statement / relational-statement

contextual-statement    = direct-contextual / inverse-contextual

direct-contextual       = peer-root-direct / inner-root-direct / entity-direct / attr-direct
peer-root-direct        = *peer-root "//" peer-root
inner-root-direct       = root-address "//" inner-root
entity-direct           = entity-address "//" entity
attr-direct             = attr-address "//" attr

inverse-contextual      = peer-root-inverse / inner-root-inverse / entity-inverse / attr-inverse
peer-root-inverse       = peer-root   "/$is()/" *peer-root
inner-root-inverse      = inner-root   "/$is()/" root-address
entity-inverse          = entity "/$is()/" entity-address
attr-inverse            = attr   "/$is()/" attr-address

literal-statement       = entity-address 1*attr "/&/" value
literal-var-statement   = entity-address 1*attr "/{&}/" value-variable
value-variable          = "{" attr-class "}"

relational-statement    = direct-relational / inverse-relational / relation-definition
direct-relational       = xdi-address "/" 1*entity "/" xdi-address
inverse-relational      = xdi-address "/$is" 1*entity "/" xdi-address

relation-definition     = direct-domain / inverse-domain / direct-range / inverse-range

direct-domain           = direct-entity-domain / direct-attr-domain
direct-entity-domain    = root-address 1*definition "/(/)/" root-address 1*definition
direct-attr-domain      = root-address *definition 1*attr-definition "/(/)/" root-address 1*definition

inverse-domain          = inverse-entity-domain / inverse-attr-domain
inverse-entity-domain   = root-address 1*definition "/$is(/)/" root-address 1*definition
inverse-attr-domain     = root-address 1*definition "/$is(/)/" root-address *definition 1*attr-definition

direct-range            = direct-entity-range / direct-attr-range
direct-entity-range     = root-address 1*definition "/(/)#/" root-address 1*definition
direct-attr-range       = root-address 1*definition "/(/)#/" root-address *definition 1*attr-definition

inverse-range           = inverse-entity-range / inverse-attr-range
inverse-entity-range    = root-address 1*definition "/$is(/)#/" root-address 1*definition
inverse-attr-range      = root-address *definition 1*attr-definition "/$is(/)#/" root-address 1*definition

xdi-address             = root-address / entity-address / attr-address / literal-address
root-address            = *peer-root *inner-root
entity-address          = root-address *entity
attr-address            = entity-address *attr
literal-address         = entity-address 1*attr "&"

peer-root               = peer-root-instance / peer-root-variable
peer-root-instance      = "(" entity ")"
peer-root-variable      = "{" peer-root-instance "}"

inner-root              = inner-root-instance / inner-root-variable
inner-root-instance     = inner-root-peer-root / inner-root-entity
inner-root-peer-root    = "(" *peer-root "/" *entity ")"
inner-root-entity       = "(" *entity "/" *entity ")"
inner-root-variable     = "{" inner-root-instance "}"

entity                  = singleton / collection / definition / variable / meta-variable
singleton               = instance / class
collection              = "[" class "]"
definition              = "|" ( singleton / collection ) "|"
variable                = "{" ( singleton / collection / definition ) "}"
meta-variable           = "{" variable "}"

instance                =  person / group / thing / ordinal
person                  = "=" [ "!" ] [ "~" ] id-string
group                   = "+" [ "!" ] [ "~" ] id-string
thing                   = "*" [ "!" ] [ "~" ] id-string
ordinal                 = "@" [ "!" ] [ "~" ] ordinal-string

class                   = reserved-class / unreserved-class / "$" / "#" / "=" / "+" / "*" / "@"
reserved-class          = "$" xdi-name
unreserved-class        = "#" [ "~" ] id-string

attr                    = attr-singleton / attr-collection / attr-definition / attr-variable / attr-meta-variable
attr-singleton          = attr-class / attr-instance
attr-collection         = "[" attr-class "]"
attr-definition         = "|" ( attr-singleton / attr-collection ) "|"
attr-variable           = "{" ( attr-singleton / attr-collection / attr-definition ) "}"
attr-meta-variable      = "{" attr-variable "}"
attr-class              = "<" class ">"
attr-instance           = "<" instance ">"

id-string               = xdi-name / xdi-scheme / encap-iri
ordinal-string          = int / other-scheme

xdi-name                = ID_Start *( ID_Continue / "_" / "-" / "." )
pct-encoded             = "%" HEXDIG HEXDIG

xdi-scheme              = uuid-scheme / cid-scheme / other-scheme
uuid-scheme             = ":uuid:" 8HEXDIG "-" 4HEXDIG "-" 4HEXDIG "-" 2HEXDIG 2HEXDIG "-" 12HEXDIG
cid-scheme              = ":cid-" 1*DIGIT ":" xdi-name
other-scheme            = ":" ( lower-alpha / DIGIT ) *( lower-alpha / DIGIT / "_" / "-" / "." ) ":" xdi-name

encap-iri               = "(" absolute-iri ")"
absolute-iri            = iri-scheme ":" 1*iri-char
iri-scheme              = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
iri-char                = safe-char / %xA0-EFFFD / pct-encoded
safe-char               = unreserved / reserved / gen-delims / safe-sub-delims
unreserved              = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved                = gen-delims / safe-sub-delims
gen-delims              = ":" / "/" / "?" / "#" / "[" / "]" / "@"
safe-sub-delims         = "!" / "$" / "&" / "(" / "*" / "+" / "," / ";" / "="

value                   = "false" / "null" / "true" / object / array / number / string
object                  = begin-object [ member *( value-separator member ) ] end-object
member                  = string name-separator value
array                   = begin-array [ value *( value-separator value ) ] end-array

begin-array             = ws %x5B ws  ; [ left square bracket
begin-object            = ws %x7B ws  ; { left curly bracket
end-array               = ws %x5D ws  ; ] right square bracket
end-object              = ws %x7D ws  ; } right curly bracket
name-separator          = ws %x3A ws  ; : colon
value-separator         = ws %x2C ws  ; , comma

number                  = [ "-" ] int [ frac ] [ exp ]
exp                     = [ "e" / "E" ] [ "-" / "+" ] 1*DIGIT
frac                    = "." 1*DIGIT
int                     = "0" / ( %x31-39 *DIGIT )   ; no leading zeros

string                  = quotation-mark *char quotation-mark

char                    = unescaped / backslash ( quotation-mark / backslash / 
                          "/" / "b" / "f" / "n" / "r" / "t" / "u" 4HEXDIG )

backslash               = %x5C           ; \ reverse solidus U+005C
quotation-mark          = %x22           ; " quotation mark  U+0022

unescaped               = %x20-21 / %x23-5B / %x5D-10FFFF 

ws                      = *( %x20 / %x09 / %x0A /  %x0D )

ALPHA                   = %x41-5A / %x61-7A                            ; A-Z, a-z
DIGIT                   = %x30-39                                      ; 0-9
HEXDIG                  = %x30-39 / %x41-46 / %x61-66                  ; 0-9, a-f, A-F
CRLF                    = %x0D / %x0A / ( %x0D %x0A )