Author: Unknown
Subject: ${a=${}}
Date: Tuesday, 20 Oct 2020, 00:00:19

When I run 
(1)$ ${a=b}
(2)$ ${a=${}}

McBash outputs two lines, each with one "b". I would expect this to be the case if the inner ${ implies the start of an expression, which is then closed by }. Does this mean that any "${" begins an expression, which is only later determined to be valid or not? I ask because my initial sense was that
${a=${}} should be expanded to b}, since the inner ${ did not begin one of the four valid expansions.