If it doesn't change inside the function, pass it as const.
Add a comment to the postscript file to say what is being done.
Lets try and stick to a (soft) line limit of 80.

Test also show that string="Text"+val.str() is slower than
string="Text";string+=val.str().
Introduced a bug in commit e830948a, fix it.

Some isotpes have a negative value of mass-excess. Moving away from
a direct check against 0 meant that dm/m was an impossibly small
value.