Omitting from $0

[edemaine]

• -Foo omits Foo from $0, and doesn't assign it $i index.
• &/! do this automatically (this would be a breaking change, potentially requiring all grammars to be rewritten, as it shifts numeric indexing, unless we still assign them $i indices)
• Alternative: ^Foo selects Foo for $0 (omitting others not marked ^)