Added introduction.

manuscript/chapter10.txt

@@ -1,42 +1,30 @@
-# Traps
+# Arithmetic
 
-Traps allow you to execute code on various signals. In `pxltrm` I'm using traps to redraw the user interface on window resize. Another use case is cleaning up temporary files on script exit.
-
-These `trap` lines should be added near the start of your script so any early errors are also caught.
-
-**NOTE:** For a full list of signals, see `trap -l`.
-
-
-## Do something on script exit
+## Simpler syntax to set variables
 
 ```shell
-# Clear screen on script exit.
-trap 'printf \\e[2J\\e[H\\e[m' EXIT
+# Simple math
+((var=1+2))
+
+# Decrement/Increment variable
+((var++))
+((var--))
+((var+=1))
+((var-=1))
+
+# Using variables
+((var=var2*arr[2]))
 ```
 
-## Ignore terminal interrupt (CTRL+C, SIGINT)
+## Ternary tests
 
 ```shell
-trap '' INT
-```
-
-## React to window resize.
-
-```shell
-# Call a function on window resize.
-trap 'code_here' SIGWINCH
-```
-
-## Do something before every command.
-
-```shell
-trap 'code_here' DEBUG
-```
-
-## Do something when a shell function or a sourced file finishes executing
-
-```shell
-trap 'code_here' RETURN
+# Set the value of var to var2 if var2 is greater than var.
+# var: variable to set.
+# var2>var: Condition to test.
+# ?var2: If the test succeeds.
+# :var: If the test fails.
+((var=var2>var?var2:var))
 ```
 
 <!-- CHAPTER END -->