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+# URI
+
+[RFC 3986](https://tools.ietf.org/html/rfc3986) -- Uniform Resource Identifier (URI)
+
+## Example
+
+        foo://example.com:8042/over/there?name=ferret#nose
+        \_/   \______________/\_________/ \_________/ \__/
+        |           |            |            |        |
+    scheme     authority       path        query   fragment
+        |   _____________________|__
+        / \ /                        \
+        urn:example:animal:ferret:nose
+
+## Elements
+
+* `IsRelative` -- flag indicating whether or not the reference is `relative reference` (as opposed to a `URI`)
+* `Scheme` -- scheme name, if reference isn't relative
+* `UserInfo` -- user information (user name, potentially with authorization information), if any is included
+* `Host` -- host name or IP address, if included
+* `Port` -- port number, if included
+* `HasPort` -- flag indicating whether or not to include port number
+* `Path` -- sequence of strings
+  * Include absolute path by making first element empty
+* `Query` -- query string, if included
+* `Fragment` -- fragment string, if included
+
+### Special disambiguation rules
+
+* `path-abempty`: For a `URI` with an `authority` component, the path must be either empty or absolute (first element empty); otherwise, it wouldn't possible to distinguish which characters belong to `host` and which belong to the path's first segment.
+* `path-noscheme`: A `relative reference` cannot have a path whose first segment contains a colon (`:`) character; otherwise, its string representation would be ambiguous: it would also match a `URI` whose `scheme` is the part of the first segment of the path before the colon).
+
+### Notes
+
+* An empty string property indicates that it isn't included in the reference.
+* A zero port number is still valid syntactically, so we need the separate `HasPort` property to indicate whether or not to include `Port`.
+* The `authority` component is only included if `Host` is not empty, regardless of whether or not `UserInfo` is empty, and regardless of `HasPort`.
+* If the first element of `Path` is an empty string, it indicates that the path is absolute.
+
+## Syntax
+
+    URI-reference = URI / relative-ref
+
+    URI           = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
+    absolute-URI  = scheme ":" hier-part [ "?" query ]
+    relative-ref  = relative-part [ "?" query ] [ "#" fragment ]
+
+    relative-part = "//" authority path-abempty
+                  / path-absolute
+                  / path-noscheme
+                  / path-empty
+    scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
+    hier-part     = "//" authority path-abempty
+                  / path-absolute
+                  / path-rootless
+                  / path-empty
+    query         = *( pchar / "/" / "?" )
+    fragment      = *( pchar / "/" / "?" )
+
+    authority     = [ userinfo "@" ] host [ ":" port ]
+    path          = path-abempty    ; begins with "/" or is empty
+                  / path-absolute   ; begins with "/" but not "//"
+                  / path-noscheme   ; begins with a non-colon segment
+                  / path-rootless   ; begins with a segment
+                  / path-empty      ; zero characters
+    path-abempty  = *( "/" segment )
+    path-absolute = "/" [ segment-nz *( "/" segment ) ]
+    path-noscheme = segment-nz-nc *( "/" segment )
+    path-rootless = segment-nz *( "/" segment )
+    path-empty    = 0<pchar>
+    pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"
+
+    userinfo      = *( unreserved / pct-encoded / sub-delims / ":" )
+    host          = IP-literal / IPv4address / reg-name
+    port          = *DIGIT
+    segment       = *pchar
+    segment-nz    = 1*pchar
+    segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
+                  ; non-zero-length segment without any colon ":"
+
+    IP-literal    = "[" ( IPv6address / IPvFuture  ) "]"
+    IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet
+    reg-name      = *( unreserved / pct-encoded / sub-delims )
+
+    IPv6address   =                            6( h16 ":" ) ls32
+                  /                       "::" 5( h16 ":" ) ls32
+                  / [               h16 ] "::" 4( h16 ":" ) ls32
+                  / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
+                  / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
+                  / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
+                  / [ *4( h16 ":" ) h16 ] "::"              ls32
+                  / [ *5( h16 ":" ) h16 ] "::"              h16
+                  / [ *6( h16 ":" ) h16 ] "::"
+    IPvFuture     = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
+    dec-octet     = DIGIT                 ; 0-9
+                  / %x31-39 DIGIT         ; 10-99
+                  / "1" 2DIGIT            ; 100-199
+                  / "2" %x30-34 DIGIT     ; 200-249
+                  / "25" %x30-35          ; 250-255
+
+    ls32          = ( h16 ":" h16 ) / IPv4address
+                  ; least-significant 32 bits of address
+    h16           = 1*4HEXDIG
+                  ; 16 bits of address represented in hexadecimal
+
+    pct-encoded   = "%" HEXDIG HEXDIG
+    unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
+    reserved      = gen-delims / sub-delims
+    gen-delims    = ":" / "/" / "?" / "#" / "[" / "]" / "@"
+    sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
+                  / "*" / "+" / "," / ";" / "="