(impl: compiler/nimconf.nim)

CfgSwitch ::= CfgVar*|EnvVar (using strtabs.%)*

ExprContainsVar ::= SwitchValExpr of CfgSwitch (e.g. release or PYTHONHOME)

SwitchValExpr ::= Tok{ & SwitchValExpr }

LhrCfgSwitch ::= CfgSwitch [ [ ident ] ]

[-|—]LhrCfgSwitch: SwitchValExpr (e.g. —hints[XDeclaredButNotUsed]:off)

(-|—)LhrCfgSwitch

[-|—]LhrCfgSwitch%=ExprContainsVar

[-|—]LhrCfgSwitch=SwitchValExpr

@if ExprContainsVar: …

[@elif ExprContainsVar: …]

[@else: …]

@end