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+* Build
+** bootstrapping
+Libraries that are part of the "bootstrap set" cannot rely on
+autoloading.  Example: =files.el= must ~(eval-when-compile (require
+'pcase))~, even though it only uses autoloaded macros.
+* Tests
+** Running
+Somehow I can never remember how to ask =make= how to run a specific
+set of tests.
+
+tl;dr, running =$testcases_regexp= from =test/$testfile.el=, to test
+changes in =lisp/$sourcefile.el=:
+#+begin_src sh
+make -C test $testfile \
+     TEST_LOAD_EL=yes \
+     TEST_BACKTRACE_LINE_LENGTH=nil \
+     SELECTOR="\"$testcases_regexp\"" \
+     EMACS_EXTRAOPT="-l ../lisp/$sourcefile.el"
+#+end_src
+
+Relevant bits from test/Makefile:
+
+#+begin_src makefile
+## filename.log: run tests from filename.el(c) if .log file needs updating
+## filename: re-run tests from filename.el(c), with no logging
+#+end_src
+
+#+begin_src makefile
+# Whether to run tests from .el files in preference to .elc, we do
+# this by default since it gives nicer stacktraces.
+# If you just want a pass/fail, setting this to no is much faster.
+export TEST_LOAD_EL ?= \
+    $(if $(findstring $(MAKECMDGOALS), all check check-maybe),no,yes)
+#+end_src
+
+#+begin_src makefile
+TEST_RUN_ERT = --batch --eval '(ert-run-tests-batch-and-exit (quote ${SELECTOR_ACTUAL}))' ${WRITE_LOG}
+#+end_src
+
+#+begin_src makefile
+ifdef SELECTOR
+SELECTOR_ACTUAL=$(SELECTOR)
+#+end_src
+
+Selector documentation from ~ert-select-tests~:
+#+begin_quote
+Valid SELECTORs:
+
+nil  -- Selects the empty set.
+t    -- Selects UNIVERSE.
+:new -- Selects all tests that have not been run yet.
+:failed, :passed       -- Select tests according to their most recent result.
+:expected, :unexpected -- Select tests according to their most recent result.
+a string -- A regular expression selecting all tests with matching names.
+a test   -- (i.e., an object of the ert-test data-type) Selects that test.
+a symbol -- Selects the test that the symbol names, signals an
+    ‘ert-test-unbound’ error if none.
+(member TESTS...) -- Selects the elements of TESTS, a list of tests
+    or symbols naming tests.
+(eql TEST) -- Selects TEST, a test or a symbol naming a test.
+(and SELECTORS...) -- Selects the tests that match all SELECTORS.
+(or SELECTORS...)  -- Selects the tests that match any of the SELECTORS.
+(not SELECTOR)     -- Selects all tests that do not match SELECTOR.
+(tag TAG) -- Selects all tests that have TAG on their tags list.
+    A tag is an arbitrary label you can apply when you define a test.
+(satisfies PREDICATE) -- Selects all tests that satisfy PREDICATE.
+    PREDICATE is a function that takes an ert-test object as argument,
+    and returns non-nil if it is selected.
+#+end_quote
+** Writing
+*** Mocking functions
+#+begin_src elisp
+(cl-letf (((symbol-function 'read-something)
+           (lambda (&rest _) "result")))
+  (some-command))
+#+end_src
+
+* Third-party packages
+** forge
+*** build
+Use =config.mk= to set =LOAD_PATH=:
+#+begin_src makefile
+find-lib = $(dir $(shell emacs -Q -batch                \
+	-eval "(require 'find-func)"                    \
+	-eval "(package-initialize)"                    \
+	-eval "(require '$(1))"                         \
+	-eval "(princ (find-library-name \"$(1)\"))"))
+
+sources = ~/src/emacs/magit/lisp
+
+elpa =                                          \
+	closql                                  \
+	compat                                  \
+	dash                                    \
+	emacsql                                 \
+	ghub                                    \
+	markdown-mode                           \
+	transient                               \
+	treepy                                  \
+	with-editor                             \
+	yaml
+elpa_dirs = $(foreach d,$(elpa),$(call find-lib,$(d)))
+
+LOAD_PATH = $(addprefix -L ,$(elpa_dirs) $(sources))
+#+end_src
+
+*** load sources
+~emacs -Q -L ./lisp~ does not seem to work: after running
+~(package-initialize)~, =M-x find-library forge= returns the instance
+installed under ~package-user-dir~.
+
+This can be circumvented with ~(setq pacakge-load-list '((forge nil)
+all))~.
+
+Full command line for easy startup:
+
+#+begin_src sh
+emacs -Q -L ./lisp \
+      -eval "(setq pacakge-load-list '((forge nil) all))" \
+      -f package-initialize \
+#+end_src