All errors generated by any function in the 
Fasta module
      are defined here. Type 
t is the union of all errors, and subsets
      of this are defined as needed to specify precise return types for
      various functions.
- `empty_line pos- an empty line was found in a position- poswhere it is not allowed.
- `incomplete_input (lines,s)- the input ended
      prematurely. Trailing contents, which cannot be used to fully
      construct an item, are provided:- linesis the complete lines
      parsed and- sis any final string not ending in a newline.
- `malformed_partial_sequence s- indicates that- scould not
      be parsed into a valid (partial) sequence value.
- `sequence_is_too_long s- indicates that- sis longer than
      allowed by `max_items_per_line.
- `unnamed_char_seq x- a- char_seqvalue- xwas found without
      a preceding header section.
- `unnamed_int_seq x- an- int_seqvalue- xwas found without
      a preceding header section.