I introduce \bcancelto{label}{term}
by modifying one of the packages routines. EDITED to support labels on \bcancelto
.
The modification to \canto@vector
to produce \cantox@vector
amounted to three things:
1) adding a -
before #4
in the \vector
call;
2) Changing the ^
superscript to a _
subscript before the \raise
; and
3) adding a -
sign before #2
following the \raise
.
Then I had to define \bcancelto
to make this macro substitution and call on \cancelto
with the substitution in place.
\documentclass{article}
\usepackage{amsmath}
\usepackage[makeroom]{cancel}
\makeatletter
% #1, #2 offset of label #6 extra width to clear arrowhead
% #3, #4 vector direction #7 superscript label style
% #5 vector width #8 superscript label
\def\cantox@vector#1#2#3#4#5#6#7#8{%
\dimen@.5\p@
\setbox\z@\vbox{\boxmaxdepth.5\p@
\hbox{\kern-1.2\p@\kern#1\dimen@$#7{#8}\m@th$}}%
\ifx\canto@fil\hidewidth \wd\z@\z@ \else \kern-#6\unitlength \fi
\ooalign{%
\canto@fil$\m@th \CancelColor
\vcenter{\hbox{\dimen@#6\unitlength \kern\dimen@
\multiply\dimen@#4\divide\dimen@#3 \vrule\@depth\dimen@\@width\z@
\vector(#3,-#4){#5}%
}}_{\raise-#2\dimen@\copy\z@\kern-\scriptspace}$%
\canto@fil \cr
\hfil \box\@tempboxa \kern\wd\z@ \hfil \cr}}
\def\bcancelto#1#2{\let\canto@vector\cantox@vector\cancelto{#1}{#2}}
\makeatother
\begin{document}
\noindent
\verb|\cancel{5y}|:
\[ x+\cancel{5y}=0\]
\verb|\bcancel{5y}|:
\[ x+\bcancel{5y}=0\]
\verb|\xcancel{5y}|:
\[ x+\xcancel{5y}=0\]
\verb|\cancelto{\infty}{5y}|:
\[ x+\cancelto{\infty}{5y}=0\]
\noindent NEW! \verb|\bcancelto{\infty}{5y}|:
\[ x+\bcancelto{\infty}{5y}=0\]
\end{document}

\xcancel{}
what you want?