a java-implemented recursive-descent interpreter
only support arithmetic and assignment operations
<calculation> = <expression> | "define" <identifier> "=" <expression> <identifier> = <alphanumericstring> <alphanumericstring> = [<alphastring>{<digitstring>}] <alphastring> = [<letter>] <letter> = 'a'|…|'z'| 'A'|…|'Z' <digitstring> = [<digit>] <digit> = '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9' <number> = {'+'|'-'}<digitstring>{'.'<digitsrting>} <expression> = <term> <expression> = <term> { '+' |'-' <term>} <term> = <factor> <term> = <factor> {'*' |'/' <factor>} <factor> = <identifier> <factor> = <number> <factor> = <identifier>|<number> '^' <identifier>|<number> <factor> ='(' <expression>')' { '^' <identifier>|<number>} <factor> ='(' <expression>')' '^' '(' <expression> ')'