This the grammar rule:
symbol = '`' (KEYW|IDENT|literal|(operator|'('|')'|'['|']'|'{'|'}'|'=')+)+
'`'
':' is not an operator: =, :, :: are not available as general operators; they
are used for other notational purposes. (From
[http://nim-lang.org/docs/manual.html#lexical-analysis-operators](http://forum.nim-lang.org///nim-lang.org/docs/manual.html#lexical-analysis-operators)
)
Hence `! :!` is valid (operator followed by operator), `! : !` is not (operator
followed by colon followed by operator). Simple. :P
