Add a post about Vim facts

doc/posts/Some-Facts-About-Vim.md

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+# Some facts about Vim
+
+Let’s relax and have some fun now! Here are a few things we've found interesting during development
+and would like to share with you.
+
+- There are no such commands as `dd`, `yy`, or `cc`. For example, `dd` is not a separate command for deleting the line,
+  but a `d` command with a `d` motion.  
+  Wait, but there isn't a `d` motion in Vim! That’s right, and that’s why Vim has a dedicated set of commands
+  for which it checks whether the
+  [command equals to motion](https://github.com/vim/vim/blob/759d81549c1340185f0d92524c563bb37697ea88/src/normal.c#L6468)
+  and if so, it executes `_` motion instead.  
+  `_` is an interesting motion that isn't even documented in vi, and it refers to the current line.
+  So, commands like `dd`, `yy`, and similar ones are simply translated to `d_`, `y_`, etc.
+  [Here](https://github.com/vim/vim/blob/759d81549c1340185f0d92524c563bb37697ea88/src/normal.c#L6502)
+  is the source of this knowledge.
+
+- `x`, `D`, and `&` are not separate commands either. They are synonyms of `dl`, `d$`, and `:s\r`, respectively.
+  [Here](https://github.com/vim/vim/blob/759d81549c1340185f0d92524c563bb37697ea88/src/normal.c#L5365)
+  is the full list of synonyms.
+
+- You can read a [post](https://github.com/JetBrains/ideavim/wiki/how-many-modes-does-vim-have) about how modes work in Vim and IdeaVim.
+
+- Have you ever used `U` after `dd`? [Don't even try](https://github.com/vim/vim/blob/759d81549c1340185f0d92524c563bb37697ea88/src/ops.c#L874).
+
+- A lot of variables that refer to visual mode start with two uppercase letters, e.g. `VIsual_active`. [Some examples](https://github.com/vim/vim/blob/master/src/normal.c#L17).
+  As mentioned [here](https://vi.stackexchange.com/a/42885/12441), this was done this way to avoid the clash with X11.
+
+- Other [strange things](https://github.com/vim/vim/blob/759d81549c1340185f0d92524c563bb37697ea88/src/ex_docmd.c#L1845) from vi:
+    * ":3"       jumps to line 3
+    * ":3|..."   prints line 3
+    * ":|"       prints current line
+
+- Vim script doesn't skip white space before comma. `F(a ,b)` => E475.
+
+- Fancy constants for [undolevels](https://vimhelp.org/options.txt.html#%27undolevels%27):
+  > The local value is set to -123456 when the global value is to be used.
+
+- Vi (not Vim) is a POSIX standard, and [has a spec](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/vi.html)! Vim is mostly POSIX compliant when Vi compatibility is selected with the `'compatible'` option, but there are still some differences that can be changed with `'copoptions'`. The spec is interesting because it documents the behaviour of different commands in a stricter style than the user documentation, describing the current line and column after the command, for example. [More details can be found by reading `:help posix`](https://vimhelp.org/vi_diff.txt.html#posix).
+
+- The Vim documentation contains many easter eggs. We encounter them occasionally, but GitHub user mikesmithgh has compiled a substantial collection [here](https://github.com/mikesmithgh/vimpromptu).