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+## Technical Choices
+
+### Modularity and Granularity
+
+In an ideal world, a program should only load the libraries it needs. Penlight is
+intended to work in situations where an extra 100Kb of bytecode could be a
+problem. It is straightforward but tedious to load exactly what you need:
+
+    local data = require 'pl.data'
+    local List = require 'pl.List'
+    local array2d = require 'pl.array2d'
+    local seq = require 'pl.seq'
+    local utils = require 'pl.utils'
+
+This is the style that I follow in Penlight itself, so that modules don't mess
+with the global environment; also, `stringx.import()` is not used because it will
+update the global `string` table.
+
+But `require 'pl'` is more convenient in scripts; the question is how to ensure
+that one doesn't load the whole kitchen sink as the price of convenience. The
+strategy is to only load modules when they are referenced. In 'init.lua' (which
+is loaded by `require 'pl'`) a metatable is attached to the global table with an
+`__index` metamethod. Any unknown name is looked up in the list of modules, and
+if found, we require it and make that module globally available. So when
+`tablex.deepcompare` is encountered, looking up `tablex` causes 'pl.tablex' to be
+required.  .
+
+Modifying the behaviour of the global table has consequences. For instance, there
+is the famous module `strict` which comes with Lua itself (perhaps the only
+standard Lua module written in Lua itself) which also does this modification so
+that global variiables must be defined before use.  So the implementation in
+'init.lua' allows for a 'not found' hook, which 'pl.strict.lua' uses.  Other
+libraries may install their own metatables for `_G`, but Penlight will now
+forward any unknown name to the `__index` defined by the original metatable.
+
+But the strategy is worth the effort: the old 'kitchen sink' 'init.lua' would
+pull in about 260K of bytecode, whereas now typical programs use about 100K less,
+and short scripts even better - for instance, if they were only needing
+functionality in `utils`.
+
+There are some functions which mark their output table with a special metatable,
+when it seems particularly appropriate. For instance, `tablex.makeset` creates a
+`Set`, and `seq.copy` creates a `List`. But this does not automatically result in
+the loading of `pl.Set` and `pl.List`; only if you try to access any of these
+methods.  In 'utils.lua', there is an exported table called `stdmt`:
+
+    stdmt = { List = {}, Map = {}, Set = {}, MultiMap = {} }
+
+If you go through 'init.lua', then these plain little 'identity' tables get an
+`__index` metamethod which forces the loading of the full functionality. Here is
+the code from 'list.lua' which starts the ball rolling for lists:
+
+    List = utils.stdmt.List
+    List.__index = List
+    List._name = "List"
+    List._class = List
+
+The 'load-on-demand' strategy helps to modularize the library.  Especially for
+more casual use, `require 'pl'` is a good compromise between convenience and
+modularity.
+
+In this current version, I have generally reduced the amount of trickery
+involved. Previously, `Map` was defined in `pl.class`; now it is sensibly defined
+in `pl.Map`; `pl.class` only contains the basic class mechanism (and returns that
+function.) For consistency, `List` is returned directly by  `require 'pl.List'`
+(note the uppercase 'L'),  Also, the amount of module dependencies in the
+non-core libraries like `pl.config` have been reduced.
+
+### Defining what is Callable
+
+'utils.lua' exports `function_arg` which is used extensively throughout Penlight.
+It defines what is meant by 'callable'.  Obviously true functions are immediately
+passed back. But what about strings? The first option is that it represents an
+operator in 'operator.lua', so that '<' is just an alias for `operator.lt`.
+
+We then check whether there is a _function factory_ defined for the metatable of
+the value.
+
+(It is true that strings can be made callable, but in practice this turns out to
+be a cute but dubious idea, since _all_ strings share the same metatable. A
+common programming error is to pass the wrong kind of object to a function, and
+it's better to get a nice clean 'attempting to call a string' message rather than
+some obscure trace from the bowels of your library.)
+
+The other module that registers a function factory is `pl.func`. Placeholder
+expressions cannot be directly calleable, and so need to be instantiated and
+cached in as efficient way as possible.
+
+(An inconsistency is that `utils.is_callable` does not do this thorough check.)
+
+