The source map provides an address space for positions in spans
that is global across the source files that are compiled together.
The source files are organized in a sequence,
with the positions of each source following the ones of the previous source
in the address space of positions
(except for the first source, which starts at the beginning of the address space).
This way, any place in any source is identified by a single position
within the address space covered by the sequence of sources;
the source file is determined from the position.