[Therion] MetaPost/Tex: variable \magdecl

Is there a way to handle the content of the \magdecl TEX-variable in METAPOST? (Therion V5.3.16) In a string or numeric? - I think, i've an syntax problem. 1.) string handling: string dec; %dec:="3.022"; % FINE! dec:=\magdecl; % ERROR p_label.top(thTEX(dec),(0,0),0,8); % >> string output 2.) numeric handling: numeric dec; %dec:=3.022; % FINE! dec:=\magdecl; % ERROR p_label.top(thTEX(decimal dec),(0,0),0,8); % >> numeric to string output