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+Requirement
+===========
+
+Some sort of constraint about the environment that can be specified and satisfied.
+
+Examples:
+* a dependency on version 1.3 of the python package "foo"
+* a dependency on the apt package "blah"
+
+Requirements can be discovered from build system metadata files and from build logs.
+
+Different kinds of requirements are subclassed from the main Requirement class.
+
+Output
+======
+
+A build artifact that can be produced by a build system, e.g. an
+executable file or a Perl module.
+
+Problem
+=======
+
+An issue found in a build log by buildlog-consultant.
+
+BuildFixer
+==========
+
+Takes a build problem and tries to resolve it in some way.
+
+This can mean changing the project that's being built
+(by modifying the source tree), or changing the environment
+(e.g. by install packages from apt).
+
+Common fixers:
+
+ + InstallFixer([(resolver, repository)])
+ + DebianDependencyFixer(tree, resolver)
+
+Repository
+==========
+
+Some sort of provider of external requirements. Can satisfy environment
+requirements.
+
+Resolver
+========
+
+Can take one kind of upstream requirement and turn it into another. E.g.
+converting missing Python modules to apt or pypi packages.