Question: Heaviside recursive

I have this equation 

 

x3l := -(1/2)*g*(f[3]*m[3]+f[2]*(m[2]+m[3])+f[1]*(m[1]+m[2]+m[3]))*tau^2/(m[1]+m[2]+m[3])+x30

 

f_i are Heaviside function of x3l, so I get  recrusive error

how can i avoid it ? 

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