Add custom snippets to COC
diff --git a/modules/neovim/coc/snippets/markdown.snippets b/modules/neovim/coc/snippets/markdown.snippets
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+++ b/modules/neovim/coc/snippets/markdown.snippets
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+# A valid snippet should starts with:
+#
+# snippet trigger_word [ "description" [ options ] ]
+#
+# and end with:
+#
+# endsnippet
+#
+# Snippet options:
+#
+# b - Beginning of line.
+# i - In-word expansion.
+# w - Word boundary.
+# r - Regular expression
+# e - Custom context snippet
+# A - Snippet will be triggered automatically, when condition matches.
+#
+# Basic example:
+#
+# snippet emitter "emitter properties" b
+# private readonly ${1} = new Emitter<$2>()
+# public readonly ${1/^_(.*)/$1/}: Event<$2> = this.$1.event
+# endsnippet
+#
+# Online reference: https://github.com/SirVer/ultisnips/blob/master/doc/UltiSnips.txt
+
+snippet dydx "The derivative" w
+\frac{dy}{dx}
+endsnippet
+
+snippet d2ydx2 "The second derivative" w
+\frac{d^2y}{dx^2}
+endsnippet