money exchange service
Suppose that $S$ be an expected money exchange service, for example, accepting deal orders in USD and returning deal orders in HKD
$x$ and $y$ be two deal orders.
$g()$ be a function that accepts a deal order and returns its deal order amount.
Towards a Metamorphic Testing Methodology for Service-Oriented Software Applications https://doi.org/10.1109/QSIC.2005.67
Property: $MR_a(x,y,S(x),S(y))\ :\ 2g(S(y))=g(S(x))$ if $g(x)=2g(y)$
Source input: $x,y$
Source output: $g(x),g(y)$
Follow-up input: $S(x),S(y)$
Follow-up output: $g(S(x)),g(S(y))$
Input relation: $x,y \Rightarrow S(x),S(y)$ where $g(x)=2g(y)$
Output relation: $2g(S(y))=g(S(x))$