[qty.expr.temp.algo] Replace "constant" with "symbol"

docs/api_reference/src/quantities.tex

@@ -873,13 +873,13 @@
 
 \begin{itemdescr}
 \pnum
-At the base level, the expression template algorithms perform their operations on constants.
-A constant is a type that is not a specialization of \tcode{To}, \tcode{per}, or \tcode{power}.
+The expression template algorithms perform symbolic computations.
+A symbol is a type that is not a specialization of \tcode{To}, \tcode{per}, or \tcode{power}.
 \begin{example}
-The dimension \tcode{dim_length}, the quantity \tcode{time}, and the unit \tcode{one} are constants.
+The dimension \tcode{dim_length}, the quantity \tcode{time}, and the unit \tcode{one} are symbols.
 \end{example}
 The algorithms also support
-powers with a constant base and a rational exponent,
+powers with a symbol base and a rational exponent,
 products thereof, and
 fractions thereof.
 
@@ -913,7 +913,7 @@
 a specialization of \tcode{power<$x$, $n$, $d$>}, then its input value is $x^{n/d}$, and
 the following also apply.
 \item
-Otherwise, a type $x$ is an input constant.
+Otherwise, a type $x$ is an input symbol.
 \end{itemize}
 \begin{example}
 The input of \tcode{derived_unit<si::kilo_<si::metre>, per<power<non_si::hour, 2>>>}
@@ -937,9 +937,9 @@
 Finally, the result is simplified:
 \begin{itemize}
 \item
-There is at most a single fraction of constants.
+There is at most a single fraction of symbols.
 \item
-There is at most a single term with a given constant.
+There is at most a single term with a given symbol.
 \item
 $x/1$ is simplified to $x$.
 \item
@@ -957,7 +957,7 @@
 \item
 If $y = 1$, returns \tcode{OneType\{\}}.
 \item
-Otherwise, if the result is a single constant, returns $y$.
+Otherwise, if $y$ is a symbol, returns $y$.
 \item
 Otherwise, returns \tcode{To<$y$>}
 after applying the following mappings and