Make a postfix operator with suppressable spacing?

Question

I would like to create a postfix unary operator that normally leaves \medmuskip between the operand and postfix operator. It's easy enough to do this using, for example,

\newcommand*{\true}{\mskip\medmuskip\mathsf{true}}

so that x$A \true x$x yields

However, the extra twist is that I would like to use a group to suppress the spacing, just as is possible for binary operators (such as \otimes). In other words, x$A{\true}x$x should yield . Bonus points for making x$\true$x (note the absence of {}) yield .

Here is a skeleton to play with:

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

\newcommand*{\true}{\mskip\medmuskip\mathsf{true}}

\begin{document}
\begin{tabular}{ll}
\verb#x$A \true x$x#  & x$A \true x$x \\
\verb#x$A{\true}x$x# & x$A{\true}x$x \\
\verb#x$\true$x#    & x$\true$x
\end{tabular}
\end{document}

Answer 1

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

\newcommand*{\true}{\relax\ifnum\lastnodetype>0 \mskip\medmuskip\fi\mathsf{true}}

\begin{document}
\begin{tabular}{ll}
\verb#x$A \true$x#  & x$A \true$x \\
\verb#x$A{\true}$x# & x$A{\true}$x \\
\verb#x$\true$x#    & x$\true$x
\end{tabular}
\end{document}