expr      := number
           | expr ws unary_op 
           | expr ws expr ws binary_op
number    := digit {digit} [ '.' {digit} ]
           | '.' digit {digit}
digit     := '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
unary_op  := "chs" | "abs" | "sqr" | "sqrt" | "sin" | "cos" | "tan"
binary_op := '+' | '-' | '*' | '/' | '^'
ws        := ' ' | '\t' | '\r' | '\n'