parser: resolve ambiguity where both prefix and primary actions are defined If both actions are defined, a primary-expression action is accepted only if the next token never starts new term. For example, parsed as primary expression: ":" # next token 'end' has no action "(:)" # next token ')' has no action ":+y" # next token '+' is infix operator parsed as prefix operator: ":y" # next token 'y' is primary expression ":-y" # next token '-' is prefix operator This is mostly the same resolution as the infix/suffix rules.