The problem occurs after editing build.XML to set debug="on" and rebuilding FOP 
with Ant and JDK1.4. (I required debug on to step into the program using 
Jbuilder6.)
FOP then crashes with a run-time exception, "No such method" when rendering an 
fo file to PostScript. The crash occurs in the file PSRenderer.java at line 697:
psString = (new StringBuffer("(").append(sb).append(") t")).toString();
where sb is a previously defined StringBuffer.

Changing this to:
psString = (new StringBuffer("(").append(sb.toString()).append(") t")).toString
();
fixes the problem.

Note that Java does not have a method StringBuffer.append(StringBuffer)
There is .append(object) which is why the program compiles but this results 
in .append(object.valueOf()) rather than .toString().
What I don't understand is why it crashes with debugging on but not when debug 
is off? I have rebuilt and run FOP several times with debugging on and off but 
always get the same result; maybe there is some optimization not present in 
debugging version.