[Boost-build] what's the syntax of multiple colon?

From: Raul (ro4tub_at_[hidden])
Date: 2006-09-20 02:50:48

lib helpers : helpers.cpp : <include>boost : : <include>. ;

lib z : : <name>z <search>/home/ghost ;
lib compress : : <file>/opt/libs/compress.a ;

lib helpers : helpers.cpp : : : <include>. ;

alias header_only_library : : : :  <include>/usr/include/header_only_library
;

lib lib : lib.cpp : : : <include>. ;
alias lib_alias ;
exe main : main.cpp lib_alias ;

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the above jam code is from
http://boost.org/boost-build2/doc/html/bbv2/advanced/builtins/targets.html

i cann't understand the syntax, could u explain it for me?
thank u very much!