Two-headed version of \xrightarrow

Question

I would like a double-headed extensible right-arrow similar to \xrightarrow from amsmath.

I tried using \xtwoheadrightarrow from extpfeil, but the result differs from \xrightarrow in three respects:

• the arrow head has a different style
• the arrow itself is longer than I would like it
• the text above the arrow is too high

What's the most straightforward way of achieving a two-headed version of \xrightarrow?

\documentclass{article}
\usepackage{extpfeil}

\begin{document}
\begin{align*}
  & \xrightarrow{ABCD} \\
  & \xtwoheadrightarrow{ABCD}
\end{align*}
\end{document}

Answer 1

I'd typeset \xrightarrow, back up a little and add \rightarrow:

\documentclass{article}
\usepackage{amsmath,amssymb}

\newcommand{\rightarrowdbl}{\rightarrow\mathrel{\mkern-14mu}\rightarrow}

\newcommand{\xrightarrowdbl}[2][]{%
  \xrightarrow[#1]{#2}\mathrel{\mkern-14mu}\rightarrow
}

\begin{document}
$A \rightarrowdbl B$

$A \xrightarrow[\beta]{\alpha} B$

$A \xrightarrowdbl[\beta]{\alpha} B$

$A \xrightarrowdbl{\text{overlong text}} B$
\end{document}

Answer 2

Same principle as in egreg's answer, only using the low level command \ooalign to place the arrows.

\documentclass{article}
\usepackage{amsmath,amssymb}

\newcommand\dhrightarrow{%
  \mathrel{\ooalign{$\rightarrow$\cr%
  $\mkern3.5mu\rightarrow$}}
}

\newcommand\dhxrightarrow[2][]{%
  \mathrel{\ooalign{$\xrightarrow[#1\mkern4mu]{#2\mkern4mu}$\cr%
  \hidewidth$\rightarrow\mkern4mu$}}
}

\begin{document}
\begin{alignat*}{3}
  A \rightarrow B    &&\qquad A \xrightarrow[C]{D} B    &&\qquad A \xrightarrow{\text{long text}} B    & \\
  A \dhrightarrow B  &&\qquad A \dhxrightarrow[C]{D} B  &&\qquad A \dhxrightarrow{\text{long text}} B  &
\end{alignat*}
\end{document}