public abstract class Expr {
	public abstract Expr apply (Expr target, double value);

	public Expr add (Expr that) {
		return new AddExpr(this, that);
	}

	public Expr mul (Expr that) {
		return new MulExpr(this, that);
	}

	public Expr sub (Expr that) {
		return new SubExpr(this, that);
	}
}

@@ -84,24 +84,27 @@
 );%0A%0A%09public 
+Add
 Expr add (Ex
@@ -106,32 +106,32 @@
 d (Expr that) %7B%0A
-
 %09%09return new Add
@@ -156,24 +156,27 @@
 %09%7D%0A%0A%09public 
+Mul
 Expr mul (Ex
@@ -224,24 +224,24 @@
 t);%0A%09%7D%0A%0A
-
 %09public 
 Expr sub
@@ -232,16 +232,19 @@
 %09public 
+Sub
 Expr sub

Clarify types of common operations in pdf.