  We consider the clustering concentration on curves for solutions to the
problem $$
  \varepsilon^2 {\mathrm {div}}\big( \nabla_{{\mathfrak a}(y)} u\big)-
V(y)u+u^p\, =\, 0, \quad u>0 \quad\mbox{in }\Omega,
  \qquad
  \nabla_{{\mathfrak a}(y)} u\cdot \nu\, =\, 0\quad\mbox{on } \partial \Omega,
$$ where $\Omega$ is a bounded domain in $\mathbb R^2$ with smooth boundary,
the exponent $p$ is greater than $1$, $\varepsilon>0$ is a small parameter, $V$
is a uniformly positive smooth potential on $\bar{\Omega}$, and $\nu$ denotes
the outward normal of $\partial \Omega$. For two positive smooth functions
${\mathfrak a}_1(y), {\mathfrak a}_2(y)$ on $\bar\Omega$, the operator
$\nabla_{{\mathfrak a}(y)}$ is given by $$ \nabla_{{\mathfrak a}(y)}
u=\Bigg({\mathfrak a}_1(y)\frac{\partial u}{\partial y_1}, \, {\mathfrak
a}_2(y)\frac{\partial u}{\partial y_2}\Bigg). $$
