Move explanation of 'equation' to chapter introduction

chapters/equations.tex

@@ -1,7 +1,15 @@
 \chapter{Equations}\label{equations}
 
+An \firstuse{equation}\index{equation} is part of a class definition.
+A scalar equation relates scalar variables, i.e., constrains the values that these variables can take simultaneously.
+When $n$-1 variables of an equation containing $n$ variables are known, the value of the $n$:th variable can be inferred (solved for).
+In contrast to a statement in an algorithm section, an equation does not define for which of its variable it is to be solved.
+% henrikt-ma: Keeping this sentence from the old glossary only because it mentions the 'instantaneous equations':
+Special cases are: initial equations, instantaneous equations, declaration equations.
+
 \section{Equation Categories}\label{equation-categories}
 
+% henrikt-ma: One would have expected this list to also describe the instantaneous equations.
 Equations in Modelica can be classified into different categories depending on the syntactic context in which they occur:
 \begin{itemize}
 \item

chapters/glossary.tex

@@ -1,14 +1,5 @@
 \chapter{Glossary}\label{glossary}
 
-\glossaryitem{equation}: part of a class definition. A scalar equation relates
-scalar variables, i.e.\ constrains the values that these variables can
-take simultaneously. When n-1 variables of an equation containing n
-variables are known, the value of the nth variable can be inferred
-(solved for). In contrast to a statement in an algorithm section, an
-equation does not define for which of its variable it is to be solved.
-Special cases are: initial equations, instantaneous equations,
-declaration equations. (See \cref{equations}.)
-
 \glossaryitem{event}: something that occurs instantaneously at a specific time
 or when a specific condition occurs. Events are for example defined by
 the condition occurring in a when clause, if clause, or if expression.