Here is an example of "readonly" not behaving the way I want.
I compiled a system with libraries

                <precompile name="vision2_pre" location="$ISE_PRECOMP\vision2.ecf" readonly="true"/>
                <library name="vision2" location="$ISE_LIBRARY\library\vision2\vision2.ecf" readonly="true"/>
                <library name="base" location="$ISE_LIBRARY\library\base\base.ecf" readonly="true"/>
                <library name="gobo" location="$ISE_LIBRARY\library\gobo\gobo.ecf" readonly="true"/>

Then I added a local `y: STRIN'.  When the compiler couldn't find STRIN
in my cluster, it proceeded to search all the vision2, base and gobo clusters
for it, but there is no chance it will be there since these libraries are
all readonly.  Meanwhile, I'm waiting for completion of a search that I know
won't find the missing class.