Defining a prefix operator with space before its argument

I want a particular spacing behavior for a custom operator. I can produce the desired spacing by setting math classes (\mathord and so forth), but I have to do it on a case-by-case basis since no single math class works for all situations. Is there a more automatic solution?

In my document, the operator $(A\smalltriangleright B)$ means "coerce from A to B". It's a prefix operator and it takes subscripts in some situations.

• When an operator is applied to an argument, there should be some space between operator and argument. Either mathbin space or mathrel space would be acceptable.

• When operators are composed using the function composition operator \circ, they should act as ordinary symbols with \circ playing the role of binary operator.

I haven't been able to get a single definition of the operator that has the right spacing in both situations:

\documentclass{article} \usepackage{amsmath} \usepackage{mathabx} \newcommand{\coerceSub}[3]{(\mathord{#1}\smalltriangleright\mathord{#2})_{#3}} \begin{document} In an operator application, the ``mathrel'' version looks better. \begin{gather} % OK \mathrel{\coerceSub uv1} x \\ % Not OK: no space between operator and operand \coerceSub uv1 x \end{gather} In an operator composition, the plain version looks better. \begin{gather} % Not OK: no space between \circ and c \mathrel{\coerceSub vw1} \circ c \\ % OK \coerceSub vw1 \circ c \end{gather} \end{document}