EVOLUTION ERROR

In APL+Win, EVOLUTION ERROR is an error message indicating a glyph or syntactic feature has been used that has no meaning at the current evolution level. It occurs at  for any feature that varies with evolution level, and at   for some syntax elements related to parentheses: bracket indexing or assignment to an unparenthesized strand, and the use of parentheses to turn a hybrid function-operator into a pure function, such as   for Replicate. EVOLUTION ERROR doesn't occur at.