howto: Fix unicode

doc/HOWTO_IMPLEMENT_A_FUNCTOR.org

 #+TITLE: How to implement a functor
 #+OPTIONS: toc:nil
 #+LATEX_COMPILER: xelatex
-#+LATEX_HEADER: \usepackage{libertine}
-#+LATEX_HEADER: \usepackage{fontspec}
-#+LATEX_HEADER: \setmonofont[Scale=0.9]{Hack}
+#+LATEX_HEADER: \usepackage{unicode-math}
+#+LATEX_HEADER: \newcommand{\Bag}{\mathcal{B}}
+#+LATEX_HEADER: \newcommand{\Dist}{\mathcal{D}}
 
 If you want to add a new basic functor to CoPaR, this document is for you. It's
 written in a tutorial style to be easy to follow along. Please refer to CoPaR's
@@ -46,7 +46,7 @@ documentation and a parser for our functor:
 simplePowerset :: FunctorDescription SimplePowerset
 simplePowerset = FunctorDescription
   { name = "SimplePowerset"
-  , syntaxExample = "PX | ƤX"
+  , syntaxExample = "PX"
   , description = Just "Some help text"
   , functorExprParser = undefined
   }
@@ -162,15 +162,15 @@ the syntax.
 
 Each functor in CoPaR has a functor expression parser, which are combined to
 parse the first line of the input file to determine the functor for the
-coalgebra that follows. The functor expression for our powerset should accept
-either =P X= or the fancy UTF-8 symbol =Ƥ X= where =X= can be anything. Functors
-with similar syntax are very common (e.g. =Ɓ= or =Ɗ=), so there's a helper
-function called ~prefix~ that makes writing parsers as easy as:
+coalgebra that follows. The functor expression for our powerset should accept =P
+X= where =X= can be anything. Functors with similar syntax are very common (e.g.
+$\Bag$ or $\Dist$), so there's a helper function called ~prefix~ that makes
+writing parsers as easy as:
 
 #+BEGIN_SRC haskell
 powersetParser :: _
 powersetParser = prefix $ do
-  L.symbol "P" <|> L.symbol "Ƥ"
+  L.symbol "P"
   return SimplePowerset
 #+END_SRC
 
@@ -180,7 +180,7 @@ With this implemented, we can now complete our ~FunctorDescription~:
 simplePowerset :: FunctorDescription SimplePowerset
 simplePowerset = FunctorDescription
   { name = "SimplePowerset"
-  , syntaxExample = "PX | ƤX"
+  , syntaxExample = "PX"
   , description = Just "Some help text"
   , functorExprParser = powersetParser
   }