(eplain.info)Arrows and morphisms

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6.2.1 Arrows and morphisms
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The macros '\mapright' and '\mapleft' produce right and left pointing
arrows, respectively. Use superscript ('^') to place a morphism above
the arrow, e.g., '\mapright^\alpha'; use subscript ('_') to place a
morphism below the arrow, e.g., '\mapright_{\tilde l}'. Superscripts and
subscripts may be used simulataneously, e.g., '\mapright^\pi_{\rm
epimor.}'.

   Similarly, the macros '\mapup' and '\mapdown' produce up and down
pointing arrows, respectively. Use '\rt' to place a morphism to the
right of the arrow, e.g., '\mapup\rt{\rm id}'; use '\lft' to place a
morphism to the left of the arrow, e.g., '\mapup\lft\omega'. '\lft' and
'\rt' may be used simultaneously, e.g., '\mapdown\lft\pi\rt{\rm
monomor.}'.

   Slanted arrows are produced by the macro '\arrow', which takes a
direction argument (e.g., '\arrow(3,-4)'). Use '\rt' and '\lft' to place
morphisms to the right and left, respectively, of the arrow. A slanted
line (no arrowhead) is produced with the macro '\sline', whose syntax is
identical to that of '\arrow'.

   The length of these macros is predefined by the default TeX
dimensions '\harrowlength', for horizontal arrows (or lines),
'\varrowlength', for vertical arrows (or lines), and '\sarrowlength',
for slanted arrows (or lines). To change any of these dimensions, say,
e.g., '\harrowlength=40pt'. As with all other TeX dimensions, the change
may be as global or as local as you like. Furthermore, the placement of
morphisms on the arrows is controlled by the dimensions '\hmorphposn',
'\vmorphposn', and '\morphdist'. The first two dimensions control the
horizontal and vertical position of the morphism from its default
position; the latter dimension controls the distance of the morphism
from the arrow. If you have more than one morphism per arrow (i.e., a
'^'/'_' or '\lft'/'\rt' construction), use the parameters
'\hmorphposnup', '\hmorphposndn', '\vmorphposnup', '\vmorphposndn',
'\hmorphposnrt', '\hmorphposnlft', '\vmorphposnrt', and
'\vmorphposnlft'. The default values of all these dimensions are
provided in the section on parameters that follows below.

   There is a family of macros to produce horizontal lines, arrows, and
adjoint arrows. The following macros produce horizontal maps and have
the same syntax as '\mapright':

'\mapright'
     '$X\mapright Y$' = (a right arrow).

'\mapleft'
     '$X\mapleft Y$' = (a left arrow).

'\hline'
     '$X\hline Y$' = (horizontal line)

'\bimapright'
     '$X\bimapright Y$' = (two right arrows).

'\bimapleft'
     '$X\bimapleft Y$' = (two left arrows)

'\adjmapright'
     '$X\adjmapright Y$' = (two adjoint arrows; left over right)

'\adjmapleft'
     '$X\adjmapleft Y$' = (two adjoint arrows; right over left)

'\bihline'
     '$X\bihline Y$' = (two horizontal lines)

   There is also a family of macros to produce vertical lines, arrows,
and adjoint arrows. The following macros produce vertical maps and have
the same syntax as '\mapdown':

'\mapdown'
     (a down arrow)

'\mapup'
     (an up arrow)

'\vline'
     (vertical line)

'\bimapdown'
     (two down arrows)

'\bimapup'
     (two up arrows)

'\adjmapdown'
     (two adjoint arrows; down then up)

'\adjmapup'
     (two adjoint arrows; up then down)

'\bivline'
     (two vertical lines)

   Finally, there is a family of macros to produce slanted lines,
arrows, and adjoint arrows. The following macros produce slanted maps
and have the same syntax as '\arrow':

'\arrow'
     (a slanted arrow)

'\sline'
     (a slanted line)

'\biarrow'
     (two straight arrows)

'\adjarrow'
     (two adjoint arrows)

'\bisline'
     (two straight lines)

   The width between double arrows is controlled by the parameter
'\channelwidth'. The parameters '\hchannel' and '\vchannel', if nonzero,
override '\channelwidth' by controlling the horizontal and vertical
shifting from the first arrow to the second.

   There are no adornments on these arrows to distinguish inclusions
from epimorphisms from monomorphisms. Many texts, such as Lang's book
'Algebra', use as a tasteful alternative the symbol 'inc' (in roman)
next to an arrow to denote inclusion.

   Future enhancements will include a mechanism to draw curved arrows
found in, e.g., the Snake Lemma, by employing a version of the '\path'
macros of Appendix D of 'The TeXbook'.