[mkgmap-dev] Syntax Question

Could someone explain to me the correct way and position to use the (!) in the style files. For example, in the current 'default style' the following two lines exist: railway=rail & !(tunnel=yes) [0x14 resolution 21] leisure=track & area!=yes What is the significance of putting the (!) before the open brackets or before the (=) sign. I mean, would there be any difference between the following two statements and would they both work?: railway=rail & !(tunnel=yes) railway=rail & tunnel!=yes Many thanks.