如何检查 MetaPost 中是否定义了宏

Question

假设在 metapost 中可能有一个具有特定名称的宏，也可能没有，并且根据具体情况，代码的行为需要以某种方式改变。如下所示：

vardef something (expr someargument) = someargument**2 enddef;
%vardef somethingElse (expr someargument) = someargument**3 enddef;

vardef doesItExist@# =
    % the code for the check goes here
enddef;

if doesItExist.something:
    fill fullcircle scaled 2cm withcolor red;
else:
    fill fullcircle scaled 2cm withcolor green;
fi; % should produce a red circle

if doesItExist.somethingElse:
    fill fullcircle scaled 1cm withcolor red;
else:
    fill fullcircle scaled 1cm withcolor green;
fi; % should produce a green circle

这能做到吗？