[Tugindia] Help! XY Package

[Abdullah Sowkaar]

Hi Friends

While compiling the following code, i am getting a black rectangular block 
over the rightarrow of the display equation which hides the whole arrow.

Can any one help me out of this

\documentclass{book}

\input xy
\xyoption{all}

\begin{document}

We suppose that the reader is familiar with the notion of a
category and of a functor between two categories. For the
reader's convenience we briefly recall a few central notions. If
not otherwise stated all functors are covariant.

Let $A$ and $B$ be two categories. A functor $F:A\to B$ is
\emph{full} if for any two objects $A,B\in A$ the induced map
$$\xymatrix{F:Hom(A,B)\ar[r]&Hom(F(A),F(B))}$$ is surjective. The functor 
$F$ is
called if this map is injective for all $A,B\in A$.

\end{document}

ThankX
abdullah

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