Here's a calculator based on google calculator that uses a buffer (the thing on the left of the >). Here is the ruby version. Type expressions and they are evaluated. If you type something like '+ 3', this will evaluate to 'buffer + 3'. If you type just 'sin' this will evaluate to 'sin(buffer)'. e.g. type '3' and then 'sin' and that will evaluate to 'sin(3)'. If there are units in the buffer, type 'u' to remove them.