examples: autojump demo

doc/examples/autojump-if-empty/README.md

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+# Autojump from tabstop when it's empty
+
+UltiSnips offers enough API to support automatic jump from one tabstop to
+another when some condition is encountered.
+
+One example of applying that behaviour is jump on the next placeholder when
+current becomes empty when user types `<BS>` or another erase sequence when
+tabstop is active.
+
+Let's imagine, that we have following snippet:
+
+![snippet](https://raw.githubusercontent.com/SirVer/ultisnips/master/doc/examples/autojump-if-empty/snippet.gif)
+
+First placeholder, surrounded by braces, can be erased by user, but then 
+surrounding quotes will left untouched, and user should remove quotes and
+one space, and only then jump to next placeholder, that equals to **5** total
+keypresses: <kbd>BackSpace</kbd> (erase placeholder), <kbd>BackSpace</kbd> and
+<kbd>Delete</kbd> (erase braces), <kbd>Delete</kbd> (erase space),
+<kbd>Ctrl+J</kbd> (jump to next placeholder).
+
+However, with UltiSnips, it can be done via only one keypress:
+<kbd>BackSpace</kbd>:
+
+![demo](https://raw.githubusercontent.com/SirVer/ultisnips/master/doc/examples/autojump-if-empty/demo.gif)
+
+## Implementation
+
+Using awesome px library, which provides set of functions to make coding
+little bit easier.
+
+```
+global !p
+import px.snippets
+endglobal
+
+global !p
+# This function will jump to next placeholder when first is empty.
+def jump_to_second_when_first_is_empty(snip):
+    if px.snippets.get_jumper_position(snip) == 1:
+        if not px.snippets.get_jumper_text(snip):
+            px.snippets.advance_jumper(snip)
+
+# This function will clean up first placeholder when this is empty.
+def clean_first_placeholder(snip):
+    # Jumper is a helper for performing jumps in UltiSnips.
+    px.snippets.make_jumper(snip)
+
+    if snip.tabstop == 2:
+        line = snip.buffer[snip.cursor[0]]
+        snip.buffer[snip.cursor[0]] = \
+            line[:snip.tabstops[1].start[1]-2] + \
+            line[snip.tabstops[1].end[1]+1:]
+        snip.cursor.set(
+            snip.cursor[0],
+            snip.cursor[1] - 3,
+        )
+endglobal
+
+context "px.snippets.make_context(snip)"
+post_jump "clean_first_placeholder(snip)"
+snippet x "Description" b
+`!p jump_to_second_when_first_is_empty(snip)
+`func (${1:blah}) $2() {
+    $3
+}
+endsnippet
+```