I have finally got to grips with l3fp's exception handling. (The main 
difficulty was realising that an exception like division_by_zero is a 
different creature from the flag fp_division_by_zero.) However, I was 
surprised to find that \fp_eval:n { ln(1) } raises the underflow flag:
\flag_height:n { fp_underflow } displays 1.

Andrew

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