[Xy-pic] Code for mid-arrows

[Richard Dybowski]

Recently, I found myself needing a new symbol for a research paper. The 
required symbol is an arrow with the head located halfway along the shaft 
of the arrow; in other words, -->-- but without the gaps. Unfortunately, 
this symbol is not present amongst the arrows provided by the AMS TeX 
collection; therefore, I devised the following code to produce it. For the 
head pointing to the right, I defined

\newcommand{\rightmidarrow}{\ensuremath{\xymatrix@1@C-4mm{\ar@{-}[r]|-{\SelectTips{cm}{}\object@{>}}&}}\xspace}

And for the head pointing to the right, I defined

\newcommand{\leftmidarrow}{\ensuremath{\xymatrix@1@C-4mm{\ar@{-}[r]|-{\SelectTips{cm}{}\object@{<}}&}}\xspace}

If anyone can think of a more eloquent way of doing this, please let me know.

The above approach is fine when used for diagrams, but one problem with the 
above code is that I cannot use it to produce the midarrow symbol as a 
subscript because the size remains the same. I have tried to create the 
subscripted version of the symbol by combining the font symbols "-" and ">" 
using the \! command (i.e. - \!\!\!\!\!\!\! >), but the result is not 
satisfactory. Part of the problem here is that the "-" is too short. Is 
there a TeX command to produce a horizontal line longer than "-"?

Richard

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Richard Dybowski, 143 Village Way, Pinner, Middlesex  HA5 5AA, UK
Tel (mobile): 079 76 25 00 92