使用 Happy New Year 进行符号运算
（以下使用 Calculator 进行计算）
>> ((H*a/p+p/y)*N/e-w)/y*e+a/r
in> ((H*a/p+p/y)*N/e-w)/y*e+a/r
out> (r*e*(N*(H*a*y+p*p)-p*y*e*w)+p*y*e*y*a)|p*y*e*y*r
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>> :expand=on
Expand on. In fraction calculation mode,
e.g., (a+b)*(c+d) will return a*c+a*d+b*c+b*d.
>> ((H*a/p+p/y)*N/e-w)/y*e+a/r
in> ((H*a/p+p/y)*N/e-w)/y*e+a/r
out> (r*e*(N*(H*a*y+p*p)-p*y*e*w)+p*y*e*y*a)|p*y*e*y*r
==> r*e*N*H*a*y+r*e*N*p*p-r*e*p*y*e*w+p*y*e*y*a|p*y*e*y*r
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