Allow for multiple contractions

[melsophos]

It would be very useful to allow for contraction of several operators with another one, as it happens often in 2d CFT with exponentials of scalar fields. It appears that this already works if the exponential is on the left:

\wick{ \c2{\c1{\e}}^{\I k_2 \cdot X_2} \I \pd \c1{X}_1^{\nu_1} \I \bar\pd \c2{X}_1^{\mu_1} \e^{\I k_1 \cdot X_1} }

On the other hand, if the exponential is on the right

\wick{ \c2{\c1{\e}}^{\I k_2 \cdot X_2} \I \pd \c1{X}_1^{\nu_1} \I \bar\pd \c2{X}_1^{\mu_1} \e^{\I k_1 \cdot X_1} }

then the result is very unpredictable:

1. If this is the first call of \wick, then it works but there is an error message Package pgf error: No shape named swick-close1 is known.
2. If \wick has already been called, the line tries to be attached to some other \wick environment, but there is no error.
   Here is one image of the second case (with some other lines of my document):