It appears that when a variable is deleted in JuMP and it is part of a quadratic term, the entire term is dropped:

```
julia> using JuMP; m = Model(); @variable(m, x[1:2]);
julia> @constraint(m, c, 2 * x[1] * x[2] == 0)
c : 2 x[1]*x[2] == 0.0
julia> delete(m, x[1])
julia> c # entire quad term deleted including x[2] and its coef
c : 0 == 0.0
```

Would it make more sense to preserve `x[2]`

and add it to the affine expression along with its coefficient as exemplified below?

```
julia> using JuMP; m = Model(); @variable(m, x[1:2]);
julia> quad = @expression(m, 2 * x[1] * x[2]) # example expression
2 x[1]*x[2]
julia> for p in keys(quad.terms) # illustration of current behavior
if p.a == x[1] || p.b == x[2]
delete!(quad.terms, p)
end
end
julia> quad
0
julia> quad = @expression(m, 2 * x[1] * x[2]) # example expression
2 x[1]*x[2]
julia> function new_delete(quad, var) # rough implementation of desired behavior
for (p, coef) in quad.terms
if p.a == var && p.b == var
delete!(quad.terms, p)
elseif p.a == var
add_to_expression!(quad.aff, coef, p.b)
delete!(quad.terms, p)
elseif p.b == var
add_to_expression!(quad.aff, coef, p.a)
delete!(quad.terms, p)
end
end
end
new_delete (generic function with 1 method)
julia> new_delete(quad, x[1])
julia> quad # x[2] if preserved with its coefficient
2 x[2]
```